As we have learned, Judaism views sexual relations between husband and wife to be not just a mitzva, but an obligation that a husband has to his wife.
The Mishna (61b) discusses the frequency with which such relations are required and concludes that depending on the husband’s job – which determines how often he is home – there are greater or lesser expectations. Thus, an unemployed man may be expected to sleep with his wife every night, while a sailor is only obligated to do so once every six months.
Our Gemara discusses the obligation of married talmidei hakhamim – individuals who have dedicated themselves to the study of Torah. Rav Yehuda quotes Shmuel as ruling that talmidei hakhamim are obligated to have marital relations every Friday night. In the Talmud Yerushalmi, Rav is quoted as pointing out that this ruling follows his interpretation of the next Mishna (64b), where we learn that talmidei hakhamim eat with their wives every Friday night, a teaching that is understood by Rav to be a euphemism for engaging in marital relations.
Rashi offers a simple explanation for the choice of Friday night. He says that the night of is a time that is set aside for rest and physical pleasure, which makes it particularly appropriate for sexual relations. The Rivan adds that according to the Gemara in Shabbat, Friday night is not a time that talmidei hakhamim would ordinarily be in the study hall, so it does not divert their attention from their studies.
The ruling that Friday night is the ideal time for talmidei hakhamim to be engaged in sexual relations is repeated numerous times in the Zohar, which offers kabbalistic explanations for this practice, going so far as to suggest that Friday night is the only time that talmidei hakhamim are permitted to have relations.