The Sages say that the habit of Torah scholars is to fulfill the mitzva of ona on Friday night (Ketubot 62b), and of them it is written, “That yields its fruit in its season” (Tehilim 1:3). Halakhic authorities add that it is a mitzva for Torah scholars to fulfill the mitzva of ona on the nights of Yom Tov and Rosh Ḥodesh (MA 240:3). There are several reasons for this. First, by fulfilling the mitzva of ona, the couple also fulfills the mitzva of oneg Shabbat (making Shabbat a delight), and it is likewise proper to be extra joyful on Yom Tov and Rosh Ḥodesh. Moreover, holy days are fitting times to fulfill the mitzva of ona, as we find that after the giving of the Torah and the dedication of the Temple, Israel fulfilled the mitzva of ona (1:6 above). In addition, on these days, Torah scholars study less Torah than usual, and therefore they are more available to fulfill the mitzva of ona in the most complete way.
The mitzva to have sexual relations on Shabbat is not limited to Torah scholars. Rather, it is a mitzva for all couples, as a fulfillment of oneg Shabbat. In the words of Shulḥan Arukh: “Marital intimacy is one of the pleasures of Shabbat” (280:1).
However, sometimes it is difficult or uncomfortable to be intimate on Friday night. People are often tired then, whether from working all week long or from Shabbat preparations, and exhaustion makes it difficult to fulfill the mitzva in the ideal way. Since the essence of the Torah commandment is for their union to be joyful, if either spouse is so tired on Friday night that they will find it difficult to truly enjoy themselves, it is preferable for them to fulfill the mitzva of ona on Saturday night or on a different night when they are not tired. Although Zohar speaks of the great value of fulfilling the mitzva of ona on Shabbat, since the essence of the mitzva is for it to be joyful, when fulfilling it on Shabbat will detract from the joy, waiting until Saturday night is preferable.