Having a festive meal on Rosh Chodesh:
It is a Mitzvah [for both men and women] to increase in foods during the [day] meal eaten on Rosh Chodesh. This can be fulfilled by making an extra dish to eat for the meal. [However some Poskim rule one is to have a festive meal of similar quality to that of Shabbos, and is not to suffice with a mere extra dish. Even if one is poor at the very least he should eat a special fruit in honor of Rosh Chodesh. This meal is considered a Seudas Mitzvah. This meal is of great importance. This meal is a Segula for long life and health and prevents depression. Although in many areas the custom is to be lenient regarding this meal, nevertheless, one is to be stringent as stated above.]
Not included in yearly budget: The money spent on foods purchased in honor of the festive meals of Rosh Chodesh is not deducted from the annual budget that one is allotted in Heaven. Those that add in expenditure in return will have Hashem add to their allocated budget.
The night meal: Some Poskim rule there is no need for one to have a special meal and add extra delicacies to the night meal of Rosh Chodesh. [Likewise, even if one adds special foods to the night meal, some Poskim rule he does no fulfill his obligation of the Rosh Chodesh meal, as this obligation must be fulfilled by the day meal of Rosh Chodesh. Other Poskim however rule that one is to have a special meal also on the night of Rosh Chodesh. Some Poskim write that one is to have three meals on Rosh Chodesh in order to recite Birchas Hamazon three times.]
Must one eat bread? There is no obligation to eat bread during the Rosh Chodesh meals. One fulfills his obligation even if he eats fruit. [However, some Poskim rule one is required to wash on bread for his Rosh Chodesh meal. Practically, it is proper for one to wash on bread in order to fulfill his obligation according to all. Some Poskim write that it is likewise proper to recite Hamotzi over two whole breads, as is done on Shabbos, in order to honor the day more than a regular weekday.]
Shabbos Rosh Chodesh: When Rosh Chodesh falls on Shabbos, one is to prepare an extra dish for the Shabbos day meal in honor of Rosh Chodesh.
Erev Shabbos Rosh Chodesh: When Rosh Chodesh falls on Erev Shabbos one is to have the festive Rosh Chodesh meal on Erev Shabbos prior to the [beginning of the] 10th [Zmaniyos] hour of the day. [Practically, this is three Zmaniyos hours prior to sunset. Thus if there are 60 minutes per Zmaniyos hour that day and sunset is at 6:00 P.M. one is to start the feast before 3:00 P.M.] If it is already past the 10th hour of the day, then it is disputed in Poskim as to whether one may still eat a meal in honor of Rosh Chodesh. [Practically, those that are careful to always eat a meal in honor of Rosh Chodesh may be lenient on this matter, although are to eat less than usual. According to all, one may certainly eat foods in honor of Rosh Chodesh if he will not wash on bread for the meal.]
It is forbidden to fast on Rosh Chodesh. [This applies even on Rosh Chodesh Sivan. Nevertheless, in prior times, it was decreed to fast on certain days of Rosh Chodesh due to the suffering the Jews incurred during that time.] It is forbidden to fast even for a few hours on Rosh Chodesh.
Yartzite: One who has a Yartzite for a parent on Rosh Chodesh is not to fast on Rosh Chodesh.
Chasan: A Chasan and Kallah who is getting married on Rosh Chodesh is not to fast on Rosh Chodesh. [They are however to fast on Erev Rosh Chodesh.] This is with exception to Rosh Chodesh Nissan, in which a Chasan and Kallah are to fast even on Rosh Chodesh. [It is disputed in Poskim if Viduiy is to be recited by the Chasan and Kallah during the Mincha Davening prior to the Chupah on Rosh Chodesh. Practically, Viduiy may be recited.]
Must one eat before midday on Rosh Chodesh?
One is to be particular to eat or drink prior to midday in order so he does not fast on Rosh Chodesh until midday. This applies even if one has no intent to actually fast.
On a two day Rosh Chodesh is one to increase in food on both days?
Some Poskim rule that by a two day Rosh Chodesh one is to have a special meal on both days. Other Poskim however rule that on a two day Rosh Chodesh one is only required to have a meal on the second day.
Sparks of Kabala:
One is required to honor and respect Rosh Chodesh [with a festive meal] just as one would be required to do so for any Holiday, as on Rosh Chodesh there is an extra radiance of the soul. The Zohar states that on this day the Jew has an extra soul just like on Shabbos.
On Rosh Chodesh the fire of Gehinnom is extinguished similar to the extinguishing of Gehinnom on Shabbos. In this aspect, the day of Rosh Chodesh is even higher than Yom Tov, as on Yom Tov the fire of Gehinnom remains. The reason for this is because both Shabbos and Rosh Chodesh were given to the Jewish people from the beginning of creation, while the Holidays were given as a result of the miracles. Due to this, one is required to increase in meals on Rosh Chodesh.
 Michaber 419/1; Tur 419; Megillah 5a; Rishonim: Orchos Chaim Rosh Chodesh; Kol Bo 43; Rokeiach 228; Yireim 227; Tosafus Brachos 49a; Or Zarua 453; Meiri Beitza 15b; based on Brachos 49a; Ran Shabbos 24 in name of Rav Yehonason; See also Shmuel 1 20/18 that Yehonason discussed with David regarding his presence by the Rosh Chodesh feast taking place in the house of Shaul; Vetzaruch Iyun why Admur 249 does not mention the law of Rosh Chodesh that fall on Erev Shabbos.
 Siddur Yaavetz; Kaf Hachaim 419/4
 M”B 419/2 based on M”A 419/1; Mor Uketzia 419; Nimukeiy Orach Chaim 422/1; Rosh Brachos 49b; Mishneh Sachir 83; See below regarding night meal
 The reason: As Rosh Chodesh is juxtaposed to the other holidays in the Torah, as the verse states “Ubeyom Simchaschem Ubimoadeichem. [Tur ibid; Kaf Hachaim 419/1] Alternatively, this is because Rosh Chodesh is a day of atonement for the Jewish people. [Orchos Chaim and Kol Bo ibid] Alternatively, this is done as a reminder to Daven the Musaf prayer. [Pirkei Derebbe Eliezer; Orchos Chaim and Kol Bo ibid] Alternatively, this is done in commemoration of the feast that took place during the times in which the moon was sanctified based on testimony. [Orchos Chaim and Kol Bo ibid]
 Elya Raba 419/1; Kneses Hagedola 419; Chayeh Adam 117/2; M”B 419/2
 Siddur Rebbe Shabsi; Rokeiach 228 “One is to eat similar to a Yom Tov”; Mishmeres Shalom 30/4 that according to Rambam that Rosh Chodesh is Biblical, one is obligated to honor it just like Shabbos; Reb Avraham Halevi regarding Minhag of Sefad, brought in Chaiy Vekayam p. 27
 Ben Ish Chaiy Vayikra 10; Kaf Hachaim 419/4
 Ashel Avraham Butchach 249 regarding Rosh Chodesh that falls on Erev Shabbos
 See Yalkut Gershoni 300 that it is for this reason that the Michaber allocated a full chapter in the Shulchan Aruch for this law. The same likewise applies for Melaveh Malka; See also Taamei Haminhagim 3/48
 Neharos Eisan 1/23 in name of Harav Tzvi of Ziditchav; Yalkut Ateres Tzvi 1 p. 172
 Beir Yaakov Chaim 4
 See Aruch Hashulchan 419 for a Limud Zechus for why the world is accustomed to be lenient regarding this meal.
 As so is recorded in all Poskim mentioned in previous footnotes.
 M”A 419/1; Taz 419/1; Darkei Moshe 242; Shelah Pesachim; M”B 419/1; Rishonim: Or Zarua 453; Minhagim Tirna p. 56; Tur 419 in name of Pesikta of Rav Kahana “All of mans food is decreed from one Rosh Hashanah to the next Rosh Hashanah with exception to the expenses of Shabbos, Yom Tov, Rosh Chodesh and Chol Hamoed” of which no budget is allocated for it on Rosh Hashanah and thus if one increases in expenditure of Shabbos and Yom Tov [Hashem] adds [to his budget].” The meals of Rosh Chodesh is omitted from Admur 242/3; 529/4; Hilchos Talmud Torah 1/7
Ruling of Admur: In 242/3 Admur rules: The Sages stated that “All of mans food and expenses is allocated on Rosh Hashanah. It is then decided as to how much income he will make on behalf of providing him food and all his other needs for all the days of that year. This however is with exception to the expenses of Shabbos and Yom Tov of which no budget is allocated for it on Rosh Hashanah and thus if one increases in expenditure of Shabbos and Yom Tov [Hashem] adds [to his budget].” [ibid] Admur here omits the meals of Rosh Chodesh from this statement of the Sages. This statement is repeated in Admur 529/4 regarding Yom Tov and Hilchos Talmud Torah 1/7 regarding tuition. In these Halachas as well the meal of Rosh Chodesh is omitted.
 M”B 419/2 based on M”A 419/1
 Rosh Brachos 49b; Mishneh Sachir 83; Nimukeiy Orach Chaim 422/1 in name of his father the Darkei Teshuva; See there for a discussion on this matter and proofs and counter proofs to this ruling; See also Aruch Laner Sanhedrin 70b for a discussion on this matter
 The reason: As the month was never sanctified at night and hence, for this reason, one who forgot Ya’aleh Veyavo is not required to repeat Shemoneh Esrei by Maariv. [Nimukeiy Orach Chaim ibid]
 Rameh Mipuno 79; Ashel Avraham Butchach Tinyana 419; Yaskil Avdi 7/11 writes that although there is no obligation to increase in the night meal, it is nevertheless a Mitzvah to do so.
 Derech Chaim of Rabbeinu Menachem Dilunzano
 Admur 188/10; Michaber 188/7; Darkei Moshe 418; M”A 188/9 and 418/1 and 424; Elya Raba 418/1; M”B 419/1; Kaf Hachaim 419/2; Rishonim: Tosafus Brachos 49a; Yireim 227; Or Zarua 453 in name of Yireim; Rokeiach 228; Meiri Beitza 15b; based on Brachos 49a “If one wants he can eat and if does not want he does not have to eat”
 Ran Shabbos 24 in name of Rav Yehonason; Or Zarua 453; Peri Chadash 419 brought in Shaar Hatziyon 419/1; P”M 419 M”Z 1; Pesikta Rabasi 1/1; Yerushalmi Brachos 7/4
 M”B 419/1; Kaf Hachaim 419/2; Seemingly this applies even according to the above Poskim who exempt one from eating bread, as even they agree that one is to increase in foods in honor of the Rosh Chodesh meal, and certainly washing for the meal is considered in act of respect.
 Kaf Hachaim ibid
 Elya Raba 419/1; Kneses Hagedola 419; Beir Heiytiv 419/1; Chayeh Adam 117/2; M”B 419/2; Ben Ish Chaiy Vayikra 2/10; Mishmeres Shalom 30/4
Other Opinions: Some Poskim rule one is to celebrate the Rosh Chodesh meal on Sunday, as is the law regarding Purim that falls on Shabbos. The reason for this is because in any event one has a festive meal on Shabbos, and hence there is no recognition for the meal of Rosh Chodesh. [Rif and Rosh in name of Yerushalmi Megillah 1/4 brought in M”A 419/1 and Taz 419/1] The Bach concludes that this is not the custom. [brought in M”A and Taz ibid] However some Poskim write that one who is careful in this matter is blessed. [Ben Ish Chaiy ibid] Some Poskim suggest that one is to continue the third meal into the night of Motzei Shabbos, in honor of Rosh Chodesh, and in this way one also fulfills the Mitzvah of Rosh Chodesh. [M”A 419/1] Some Poskim rule that when Rosh Chodesh falls on Shabbos and Sunday one is to follow the advice of the M”A ibid and continue the third meal of Shabbos into Motzei Shabbos, as well as have a festive meal the next day on Sunday. [Nimukeiy Orach Chaim 419/1] Other Poskim rule one is to increase in the Melaveh Malka meal in honor of Rosh Chodesh which was that day. [Siddur Yaavetz brought in Ben Ish Chaiy ibid and Kaf Hachaim 419/3]
 Admur 249/6 regarding a Seudas Mitzvah that falls on Erev Shabbos and 249/9 regarding a small Seuda
 Some Poskim rule one may eat the Seudas Rosh Chodesh on Erev Shabbos just like any other Seudas Mitzvah which falls on Erev Shabbos. [Ashel Avraham of Butchach 249] However other Poskim rule one is not to do so. [Aruch Hashulchan 249/7]
 This matter is dependent on whether it is considered a Seudas Mitzvah, of which we rule may be eaten even past the 10th hour in a time of need. [Admur 249/7] Seudas Rosh Chodesh is not listed in Admur ibid or any of the Poskim as a Seudas Mitzvah. [See Beis Lechem Yehuda and Gilyon Maharsha on Y.D. 391] However, logically, it should be no different than Seudas Chanukah which is listed as a Seudas Mitzvah, even though its Seuda is under dispute in 670. Practically, the Ashel Avraham of Butchach 249 rules one may eat the Seudas Rosh Chodesh on Erev Shabbos just like any other Seudas Mitzvah which falls on Erev Shabbos; however the Aruch Hashulchan 249/7 rules one is not to do so.
 See Piskeiy Teshuvos 249/4
 Admur 249/9
 418/1; Mishneh Taanis 15b
Is this prohibition Biblical or Rabbinical? It is disputed amongst Rishonim as to whether the fasting prohibition on Rosh Chodesh is Biblical or Rabbinical. [See Michaber 418/4 and 570/1; M”A 418/1; Biur Halacha 418 “Rosh Chodesh Assur”; Kaf Hachaim 418/13] Some Poskim rule the Holiday of Rosh Chodesh is of Biblical status. [Implication of Rambam, brought in Michaber and Poskim ibid regarding fasts] Other Poskim rule it is only of Rabbinical nature. [Michaber ibid; M”A 418/1; See Biur Halacha 418 “Rosh Chodesh Assur”]
One who made a Neder to fast on Rosh Chodesh: See Michaber 418/2-5; 572/2 and Nosei Keilim there; Likkut Dinei Rosh Chodesh 4/3-4
Taanis Chalom: It is permitted to fast a Taanis Chalom on Rosh Chodesh. [Michaber 418/5; See Admur 288/3]
 M”A 418/1; Rameh 79
 The congregation of Worms would fast on Rosh Chodesh Sivan due to a terrible decree that occurred during that day in the year 5856. [Rokeiach 212; Beis Yosef 579]
 This is called a Taanis Shaos. This refers to one who intends to fast [for atonement and the like] for a certain amount of hours. See 562/10-11 for details of the laws of a Taanis Shaos and its exact definition. If however one does not intend to fast, and simply delay’s eating due to side factors, then it is not forbidden to do so. [Kaf Hachaim 418/3]
 M”A 418/1 states that this matter is a dispute; Biur Halacha 418 “Rosh Chodesh Assur” concludes it is forbidden according to all. So is also implied from P”M 418 A”A 1 who questions this ruling of the M”A; Kaf Hachaim 418/3 concludes in name of Achronim that even a Taanis Shaos is forbidden.
 Michaber 568/9
When is he to fast? Some Poskim rule he is to fast on the day after Rosh Chodesh. [Michaber ibid; Dvar Moshe 68; Ruach Chaim 568/7; Zechor Leavraham] Others rule he is not to fast at all. [Rama ibid] Others rule he is to fast on Erev Rosh Chodesh. [Pnei Ahron 27; Kaf Hachaim 568/94]
 See Michaber and Rama 573/1; M”A 573/1
How much should he eat? Some record the Rebbe to have given a directive to a certain Chasan that he is to drink a strong drink that day, but not to eat. [See Seder Kiddushin Venisuin-Farkash p. 34]
 Igros Kodesh 22/152 [printed in Shulchan Menachem 6/190]; Toras Yekusiel 141/98; See Nitei Gavriel 6/10; Hiskashrus 643/17; Igros Kodesh Rayatz 17/394
 Admur 429/9; Rama 573/1
The reason: It is permitted for a Chasan to fast on Rosh Chodesh Nissan being that some say one is to always fast on this date, as on Rosh Chodesh Nissan Nadav and Avihu passed away. [Rama ibid; Michaber 580/1-2]
 See Nitei Gavriel Nissuin 8/8 that some Poskim rule it is to be said [Kerem Shlomo 65/14, Levushie Mordechai E.H. 49 and other Poskim], while other Poskim rule it is not to be said. [Yalkut Gershoni 2/11; Shevilei David 62; Toras Yekusiel 42]
 Nitei Gavriel ibid in name of Yad Halevi 3/158
 Poskim ibid that one may not fast a Taanis Shaos [however no mention is made regarding midday or regarding one who does not intend to fast]; Admur 288/1 rules that it is forbidden to fast until midday on Shabbos, even if one does not intend to do so as a fast. The same applies towards Yom Tov and Chol Hamoed. [M”A 652/4; M”B 652/7 regarding Chol Hamoed Sukkos] Seemingly then the same would apply to Rosh Chodesh. However regarding Rosh Hashanah see Ateres Zekeinim 597; M”B 597/2; Kaf Hachaim 597/6
 Pnei Moshe Yerushalmi Taanis 4/3; Mateh Moshe 530
 Yosef Ometz 694
 Or Tzaddikim 31/1; Machazik Bracha 419/1; Kaf Hachaim 419/5
 See Zohar 81b; Tikkunei Zohar 421
 Birkeiy Yosef 419
 Mordechai Pesachim 166 in name of Pesikta