Sefirat Ha-Omer ספירת העומר

(15) And from the day on which you bring the sheaf of elevation offering—the day after the sabbath—you shall count off seven weeks. They must be complete: (16) you must count until the day after the seventh week—fifty days; then you shall bring an offering of new grain to the LORD.

(3) The Feast of Weeks is the anniversary of the Revelation on Mount Sinai. In order to raise the importance of this day, we count the days that pass since the preceding festival, just as one who expects his most intimate friend on a certain day counts the days and even the hours. This is the reason why we count the days that pass since the offering of the Omer, between the anniversary of our departure from Egypt and the anniversary of the Lawgiving. The latter was the aim and object of the exodus from Egypt...

Why does the Torah say to bring the Omer on Pesach? Because Pesach is the time of field produce. God said, 'Bring an Omer berfore me at Pesach, in order that the field produce be blessed for you.'…

(24) Every Jew is required to perform this duty [of sefirat ha-omer], everywhere and at all times.

Could it be that one would cut the omer during the day and count during the day and bring the offering during the day? The verse teaches "seven complete (temimot) weeks they will be," When are they temimot? At the time when it [the count] begins—at night. Could it be that one would cut at night and count at night and bring [the offering] at night? The verse teaches "from the day of your bringing." Bringing [the offering] is only during the day.

From the beginning of the day we count. Therefore one should count at night, from the night of the sixteenth of Nissan. If one forgot and did not count at night, he counts during the day.

Rabbeinu Tam says that if he forgot to recite a beracha at night, he should not recite a beracha during the day….for it is written "temimot," complete [days]. You only find temimot when you count at night [the onset of the halachic day]…Behag wrote that where one forgot to recite a beracha at night, he should count the next day without a beracha, and this is the halacha. But if he forgot a night and day, he should not count any further with a beracha, because we require temimot and do not have them.

(7) If he forgot [to count] and did not do the blessing all night, he should count during the day without a blessing.

(8) If he forgot to bless on one of the days, between the first day [of the omer] and the rest of the days, he should count the rest of the days without a blessing...

(24) Every Jew is required to perform this duty, everywhere and at all times. Women and slaves, however, are exempt from it.

He [the sage in the mishna listing mitzva categories] taught some cases and left others out….and regarding a positive commandment that is not time-bound, he left out a lot, [including] …sefirat ha-omer.

For the omer was already offered, and they count from it.

Because these days are days from the realm of the masculine, this count was only given over exclusively to men.

Women are exempt from counting [the omer] because it is a positive time-bound commandment… Nevertheless, they already accepted it upon themselves like an obligation.

It seems that in our lands women are not at all accustomed to count.

Women are exempt because it is a positive time-bound commandment. In any case, they have been accustomed to recite a beracha and to count, like any positive time-bound commandment that women practice, like shofar and sukkah and lulav.

(2) It is customary not to cut one's hair until Lag BaOmer, since it is said that that is when they stopped dying. One should not cut one's hair until the 34th day, in the morning, unless the 33rd day falls on Friday, in which case one may cut one's hair then, in honor of the Sabbath ("kavod Shabbat"). Rema: But in these countries, we do not follow the custom he advocates; rather, we cut our hair on the 33rd day...

Rav Hai wrote in a responsum, that which we have the custom not to perform labor from Pesach till Shavu'ot from sunset till the morning prayer is because of the students of Rabbi Akiva, for all of them dies after sunset and were buried after sunset and the people were taken away from labor. Further that it is written seven complete weeks [Shabbatot] they will be, from the language of shevut [desisting from labor] and the sabbatical year, as it is written: seven sevens of years. Just as a sabbatical year is prohibited in agricultural labor, so too the time of counting the omer, which is after sunset, one should desist from labor.

They [women] only desisted from labor after sunset until the evening prayer, after the count. … Even now there are women who maintain this custom.

(4) Women have the custom not to do work from Pesach until Shavu'ot, from sunset onwards.

They [women] are rewarded with acclaim in every generation, for they were the first to come to those [deceased] students and honor them and take care of them. Therefore they [women] were accustomed not to perform labor at that time as a commemoration of the righteous women of that generation.