Variantly: "on the fifteenth day of the second month": Why is "day" mentioned? To know on which day the Torah was given to Israel. (Rosh Chodesh of the) Nissan on which Israel left Egypt fell out on the fifth day of the week. Nissan was a complete month (thirty days, so that Rosh Chodesh) Iyyar fell out on the Sabbath. Iyyar was a defective month (twenty-nine days, so that Rosh Chodesh) Sivan fell out on the first day of the week. And it is written (Numbers 33:3) "On the morrow of the Pesach, the children of Israel went out," and (here) "on the fifteenth day of the second month," and (Exodus 19:1) "On the third month of the exodus of the children of Israel, they came to the desert of Sinai," whence it is derived (that the sixth day of their encampment was on the third month (Sivan), on the sixth day of the month, on the eve of the Sabbath, (and the Torah was given the next day, Sabbath [viz. Shabbat (the Sabbath)h 87b-88a]).