Halakha Imamat 23:15: Kitzur Shulchan Arukh, Mishneh Torah, Positive Mitzvot & Sefer HaChinukh

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).

To count forty-nine days from the time of cutting the Omer (first sheaves of the Barley Harvest), as it is said, “And ye shall count unto you from the morrow after the Sabbath (feast)” (Lev. 23:15).