[To answer a question about the timing of the giving of the Torah at Sinai, the gemara posits (according to one opinion) that] Moshe added, on his own, a day of separation from wives. This is as it says in a baraita: Moshe did three things of his own volition, and Hashem agreed with him.
"R' Yochanan said: Three keys the Holy One blessed be He has retained in His own hands and not entrusted to the hand of any messenger, namely, the Key of Rain, the Key of Childbirth, and the Key of the Revival of the Dead"