Counting of the Omer ספירת העומר

The commandment is strange in its simplicity: each evening say which day of the count it is. "And you shall count for yourselves from the morrow after the rest day… seven complete weeks shall there be." It begins with the barley offering of the omer and ends with the two loaves of new wheat on Shavuot.

The meaning of the count is explained thus: the people came out of Egypt but were not ready to receive the Torah, and the forty-nine days are the preparation, counted not backwards but forwards, towards the goal. That is why one says "today is such-and-such a day", not "so many remain."

Part of these weeks are kept as days of mourning: by the Talmud, in them died twenty-four thousand disciples of Rabbi Akiva, "because they did not treat one another with respect." Hence the custom not to cut hair, not to marry and not to hold celebrations—with different periods in different communities.