The Halachic obligation to celebrate the day of a miracle Chanukah and Yud Tes Kisleiv
A day in which a miracle occurred is to be commemorated throughout the generations. This celebration is not a mere matter of thanks but according to some Poskim carries with it a Biblical obligation. There are laws and customs associated with this day of celebration such as making meals and omitting Tachanun. The following will clarify some of the Yud Tes Kisleiv annual customs from a Halachic perspective!
The Biblical obligation to commemorate the day:
Some Poskim rule that all holidays instituted to commemorate a miracle of G-d have Biblical status, and hence the commemoration of Chanukah and Purim are of Biblical origin. Thus, if one passes the Holiday without performing any form of commemoration of the miracle, then he has transgressed a Biblical command according to this opinion. Nevertheless, even according to this opinion, the form of how to commemorate the miracle [i.e. lighting candles, Mishloach Manos] is only Rabbinical, and hence the lighting of the candles, and other matters involved in the form of commemoration, are Rabbinical according to all. Nevertheless, even these Rabbinical forms of commemoration are of Biblical origin, as the Torah commands one to adhere to the words of the Sages and not swerve from their words. [According to the above approach, the celebration of Yud Tes Kisleiv which celebrates a miracle that Hashem performed for the Alter Rebbe and the entire Chassidic movement, and continuity of Chassidus, has Biblical status, and fulfills a Biblical Mitzvah.]
Making a meal of commemoration-Seudas Hodaya:
It is customary to celebrate a miracle through making a festive meal for one’s friends and family. During this meal one recounts the miracle that occurred and gives thanks to Hashem for His great kindness. [The Alter Rebbe celebrated a Seudas Hodaya for his release from imprisonment on Yud Tes Kisleiv, on the 19th of Kisleiv 1801/5561, two years after his release from prison. The Alter Rebbe treated this meal as an absolute obligation, and a Halachic discussion ensued with his brother the Maharil regarding what one is to do if he did not make the Seudas Hodaah on time.]
Seudas Mitzvah: A meal that takes place to commemorate a miracle performed by Hashem is considered a Seudas Mitzvah.
Not to say Tachanun:
It is customary not to say Tachanun on the day which one celebrates the miracle that took place. [For this reason Tachanun is not recited on Yud Tes-Chaf Kisleiv. The Rebbe stated that on the day of Yud Tes Kisleiv there is a great weakness in the attribute of judgment and it is therefore a Yom Segula to make good resolutions regarding matters of Torah, Mitzvos, and good character traits. Accordingly, one who recites Tachanun on this day arouses the attribute of Divine retribution. Therefore, Tachanun is not to recite on this day even by an individual, in a Shul that is accustomed not to say it, as one who does so causes damage not only to himself but also to the entire Minyan that is Davening with him.]
 See Sdei Chemed Mareches Pei 29; Piskeiy Teshuvos 670:3
 Chasam Sofer Yoreh Deah 233 based on Megillah 14a “The establishing of a Holiday on the day that a miracle occurred is a Kal Vachomer Deoraiysa, and according to my opinion [Leaniyas Daati] according to this the days of Chanukah and Purim are Biblical. However, regarding which actions to perform on this day, it is only Rabbinical. However, one who transgresses and does not perform any form of commemoration of the miracle of Chanukah or Purim, transgresses a positive Biblical command of the Torah.”; This ruling is also repeated in his Sefarim: Chasam Sofer O.C. 208; Chidushei Shabbos 22a, 23a; Hagahos Hashas Megillah 6b; Chidushei Gittin 36b
Other opinions: Some Poskim argue that while it is a Biblically command to commemorate the miracle on the day of its occurrence, this only applies by its first year, while by all other subsequent years it is only Rabbinical. [Emek Sheila Sheilasah 26:1; Arugas Habosem O.C. 181; Beis Levi 4]
 The reason: This is learned as a Kal Vachomer from the Mitzvah to tell over the story of the exodus and its miracles, from which we learn a general Mitzvah to commemorate days of miracles. [See Chasam Sofer ibid; Megillah 14a who wanted to learn from this a Kal Vachomer to require reading the Megillah]
 Chasam Sofer ibid “One who transgresses and does not perform any form of commemoration of the miracle of Chanukah or Purim, transgresses a positive Biblical command of the Torah. However, one who performs any remembrance, irrelevant of form, during one of the days of Chanukah, even if he does not light candles, and does not send Mishloach Manos on Purim, he is considered a transgressor of a Rabbinical command.”
 Chasam Sofer ibid
 Shabbos 23a; Admur 158/16
 Shelah; Chavos Yair 70; P”M 444 M”Z 9; Chayeh Adam 155/42; Shulchan Hatahor 219/7; Soles Belula 218/2; Kaf Hachaim 218/9
 I have not found any source in Poskim for this meal being obligatory, and on the contrary the above Poskim all write it is a mere custom. However from Sefer Hasichos below it is implied that it is an obligation.
 Sefer Hasichos 1940 p. 54-58 [Hebrew], retold by the Rebbe Rayatz in name of Rav Issac Homlir.
 Chavos Yair 70; P”M 444 M”Z 9; Chayeh Adam 154/42 in name of Rashal Baba Kama 37; M”B 697/2
Other Poskim: Some Poskim rule that a Seudas Hodaya is not considered a Seudas Mitzvah. [Peri Chadash 496]
 Lev Chaim 150; Kaf Hachaim 218/9; Tzitz Eliezer 10/10
 See Ketzos Hashulchan 24 footnote 30
 Likkutei Sichos 30:289
 Rebbe ibid; See Levush 131; Ateres Zekeinim 131:3; M”B 131:20