We begin to count the omer on the night of the sixteenth of Nisan, as it says, "You shall count off seven weeks; start to count seven weeks from when the sickle is first put to the standing grain" (Devarim 16:9-10). "From when the sickle is first put to the standing grain" refers to the omer harvest, so called because the first harvest of the yearly produce is designated for the omer offering. The omer harvest took place on the night after the first day of Pesaĥ, which is the sixteenth of Nisan. As a result, that is when we begin to count the omer.

One must count at night, because the Torah says regarding sefirat ha-omer, "seven weeks; they must be complete" (Vayikra 23:15), meaning that the weeks must be whole. On the Hebrew calendar, a full day consists of night followed by day, so in order to include all the nights and days of seven complete weeks, we must start counting on the night of the sixteenth of Nisan (Menaĥot 66a). So that the count covers all the hours of the day, it is a mitzva to count early, at the beginning of the night. We are especially meticulous on the first night, so that the count will cover every hour of the seven weeks. Nonetheless, we enhance the mitzva by counting at the beginning of the night on every night, so that each day’s count will be whole, covering an entire 24-hour day.

Even though it is a mitzva to count at the beginning of the night, one is not obligated to do so. Therefore, one who is about to pray Ma’ariv must do so before sefirat ha-omer, due to the principle that more frequent mitzvot take precedence over less frequent ones. The mitzvot of reciting the Shema and praying Ma’ariv apply all year round, making them more frequent than sefirat ha-omer (Ĥok Ya’akov; see bhl 489:1, s.v. "aĥar").[4]