May one have marital relations during daytime?
A Torah Scholar: It is permitted for a Torah scholar [who is modest in his ways and will not look at his wife’s private area] to have relations during the day [even in a non-dark room] so long as he darkens [the area] with his sheet [and shuts off all the lights in the room]. [However, this allowance for a Torah scholar only applies if it is a time of great need that he cannot overcome his urge. Alternatively, it applies if he could not do the Mitzvah of Onah at night for whatever reason, or if he suddenly had to travel out of town that day. Some Poskim, however, rule that in the case that one cannot overcome his urge, it is permitted for all people and not just a Torah scholar.]
Ruling of Kabbala: The above is based on Halacha, however, based on Kabala, some Poskim rule that one is to never have marital relations during daytime even in a dark room and even if one is a Torah scholar [and it is a time of need as stated above], as one who has relations during the day draws down from only reincarnated souls. Other Poskim, however, do not mention a need to abstain from marital relations during the day according to Kaballah. Practically, if delaying the matter can lead to Zera Levatala, or one’s wife desires him to fulfill his Mitzvah of Onah, then one may do it during the day, in a dark room, as stated above.
It is forbidden to have marital relations during daytime unless the room is dark, in which case it is permitted even initially although some are stringent due to reasons of Kabala. In a time of need, a Torah scholar may have intercourse even in a non-dark room if he covers himself with a sheet to darken the area.
When does day begin regarding the above law, Alos or sunrise?
The day begins as soon as the sky begins to light up, which is the time of Mi Sheyakir. Patsd this time, one may only have relations in a dark room, as stated above. However, some Poskim rule that one may even initially have relations prior to sunrise if he darkens the area with his sheets, even if he is not a Torah scholar.
How dark must the room be?
Seemingly, the room must be completely dark without any light penetrating from the outside. However, perhaps it suffices for it to be dark enough to make it difficult to see the body parts of one’s wife. Vetzaruch Iyun!
May one perform the preparatory acts to intercourse [i.e. hug, kiss, etc] during the day if the room is not dark?
Yes. The prohibition only applies to actual penetration and not to other acts that are done in preparation for the intercourse.
 Michaber O.C. 240:11; E.H. 25:5; Tur 240:11; Rambam Issurei Biya 21:10; Rav Chisda in Nida 17a “It is forbidden to have relations during the day”; Rav Huna in Shabbos 86a and Nida 17a “The Jewish people are holy and do not have relations during the day”; Kesubos 65b; See Megillah 13a “From here we learn Achashveiroshes praise, that he did not have relations during the day.” Zohar 1:49 “It is forbidden for a man to have relations during the day”; 140b “The Jewish people are holy and do not have relations during daytime”; 3:225b “It is the practice of the sages to only have relations during the night, in a way of modesty.”; Menoras Hamaor Ner 3 Klal 6 Chelek 5 Chapter 3:5; Taharas Yisrael 240:74
 The reason: It is forbidden due to being brazen and immodest and contrary to the guidelines of holiness. [Michaber E.H. ibid “It is Derech Azus”; Rambam ibid; M”B 240:44; Shaar Hatziyon 240:27; Rav Huna in Shabbos and Nida ibid “The Jewish people are holy”] Alternatively, it is forbidden as perhaps he will see something distasteful about his wife and thus come to transgress “Veahavta Lireiacha Kamoacha.” [Rav Chisda in Nida ibid; Kaf Hachaim 240:79]
 M”A 240:26 that so is implied from Kesubos 65b; Beir Heiytiv 240:21; P”M 240 A”A 26; Implication of Torah Leshma 69; However see Birkeiy Yosef 240:17, brought in Kaf Hachaim 240:80, who discredits the proof of the M”A ibid
Not to overindulge: One should not engage in marital relation by both night and day on a steady basis. [See Tosafus in Sukkah 52b; Sanhedrin 107a]
 Michaber O.C. ibid [omitted in Michaber E.H. ibid]; Rava in Shabbos ibid and Kesubos ibid; Menoras Hamaor Ner 3 Klal 6 Chelek 5 Chapter 3:5; Taharas Yisrael 240:74; Torah Lisham ibid; All Poskim in previous footnote;
Other opinions: It is implied from some Poskim that it is always forbidden to have marital relations during daytime [even if the room is dark] as this is considered brazen. [Implication of Michaber E.H. ibid, Rambam Issurei Biya 21:10 and Zohar ibid who do not differentiate between a dark room thus implying it is forbidden in all cases; See Torah Leshma ibid who learns this way in Zohar]
 Rama ibid; Tur 240; Rambam ibid; Rava in Shabbos ibid
 M”A 240:25; Tur 240; M”B 240:45
 M”A 240:25 in name of Rashi; Taharas Yisrael 240:74; M”B 240:45
 P”M 240 A”A 25
 The reason: The reason it is permitted specifically for a Torah scholar is because we trust that he will take precautions to properly darken the area, and because he does not have time to have relations at night due to his learning. [Menoras Hamaor ibid]
 M”A 240:26 in name of Bach and Rambam Issurei Biya 21; M”B 240:44; See Menoras Hamor ibid; See gloss of Chochmas Shlomo ibid who learns from Kesubos ibid that it is permitted even initially, however he then concludes like Rambam
 Menoras Hamaor ibid; Taharas Yisrael 240:74
 Shaar Hatziyon 240:28 in implication from Chochmas Adam; Taharas Yisrael 240:74
 Peri Eitz Chaim 16:11 “One who has relations during the day draws down from only reincarnated souls.”; See Torah Leshma 69 in name of Kabala Kuntrus that it is forbidden to have relations during Shabbos day, however, he questions its authenticity and concludes that one who follows Kaballah it is better for him to avoid it
 Kaf Hachaim 240:79; Piskeiy Teshuvos 240:26
 Piskeiy Teshuvos ibid such as if one did not fulfill the Mitzvah of Onah at night for whatever reason, as is common on the night of a wedding, or if one is traveling
 See Menoras Hamaor ibid; Taharas Yisrael 240:74 who all write of the above allowances of a dark room and Torah scholar and do not mention any need to initially avoid it; See Torah Leshma ibid who permits having relations during the day in a dark room to fulfill Onah if his wife desires, and concludes that even based on the Zohar it is simply improper to have relations during the day and is not forbidden; See also Or Tzadikim 32:4 [written by student of Arizal] who explicitly writes that when the night of Tevila falls on a night that relations is not done, one is to wait until day to have relations.
 See Torah Lishma ibid
 Ben Ish Chaiy Vayeira 2:26; Ashel Avraham Butchach 240; Piskeiy Teshuvos ibid
 Taharas Yisrael 240:74; Ashel Avraham
 See Admur 433:6 who describes a dark room in which it appears the same by both day and night
 So is implied from Kesubos ibid, as well as the ruling regarding Mi Sheyakir