Halakhah su Levitico 23:15

It is the custom that no work is performed, by either men or women during the Sefirah days, from sunset, until after the counting of the Omer. There is an allusion for this (in the Torah), because it is said, "Seven weeks" (Leviticus 23:15). [The word shabbasos, meaning weeks,] is derived from shevos, denoting rest, indicating that during the time we count the Omer, that is from sunset on, you should rest [refrain] from doing any work, until after you have counted the Omer.

On the first night of Shavuos, the Maariv service is delayed, until the stars come out, for, if we would hold the service before this time, and usher in the Yom Tov, a bit of time will be lacking from the forty-nine days of the Sefirah, and the Torah says, "They shall be seven complete weeks" (Leviticus 23:15).

The commandment of counting the omer: To count forty-nine days from the bringing of the omer which is on the sixteenth day of Nissan, as it is stated (Leviticus 23:15), "And you shall count for yourselves from the morrow of the Shabbat from the day of your bringing the omer of waving." And this tallying is an obligation, and it is upon us to count the days on each day and, likewise, the weeks. As Scripture said to count fifty days and also said (Deuteronomy 16:9), "Seven weeks shall you count for yourself." And in explanation, Abbaye said in the Gemara in Menachot 66a, "It is a commandment to tally the days and it is a commandment to tally the weeks" (see Sefer HaMitzvot LaRambam, Mitzvot Ase 161). And there are some of the commentators (see the end of Ran on Pesachim) the opinion of which is that the intention of the verse is to tally the weeks specifically when they are full, but there is no need to mention [them] every day and say that they are such and such days and such and such weeks. And there are some that say (Rosh in his Responsa 24:13) that the [proper] way is to mention the tally of weeks with the days always on every day. And one who fears the Heavens will choose their way to remove [himself] from any doubt, and not be concerned about the elegance of the words. And so have they practiced today in all places of which we have heard.