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The Mitzvah of Counting the Omer
1) In the Temple Era, when the Beis Ha-Mikdash stood and the Omer offering was brought, there was a Torah mitzvah to count the Count of the Omer, which spans from the bringing of the offering until the festival of Shavuos, as the verse states: “You shall count for yourselves from the morrow of the Sabbath, from the day on which you brought the Omer offering – seven complete weeks they shall be – until the morrow of the seventh week, you shall count fifty days.”
2) In times when the Temple does not stand, and the Omer offering is not brought, rishonim dispute concerning whether the Torah mitzvah of counting the Omer applies, or whether it is a rabbinic obligation alone.
3) Most authorities maintain that in the absence of the Temple, the mitzvah is only a rabbinic enactment. Nevertheless, one must follow stringencies based on the opinions that counting is a Torah mitzvah.
4) Rishonim also dispute whether or not a distinct mitzvah of counting the Omer applies each day, or whether there is one mitzvah that spans the entire length of the count.
The Time of Counting the Omer: From When to Count
1) The Count of the Omer begins on the night of the 16th of Nissan, and continues until the eve of Shavuos. The Shehechianu blessing is not made on the first night of counting.
2) One should count immediately upon nightfall. If one failed to do this, one may count with a blessing for the duration of the night.
3) One should preferably not count during the bein ha-shemashos period (after sunset but before nightfall). If one counted during the bein ha-shemashos period, one should count again, without reciting a blessing, after nightfall.
4) If a person counted in the bein ha-shemashos period, and did not count again after nightfall, one may nevertheless continue to count for the remainder of the count with a blessing.
5) In one prays in a shul where the Arvis prayer is recited before nightfall, and the Omer is counted early, one should count together with the congregation, and make a stipulation (by heart) that if he remembers to count again after nightfall, the counting should not fulfill his obligation to count. This stipulation allows one to count again after nightfall with a blessing. If he did not make this stipulation, he must nonetheless count again after nightfall, but cannot recite a blessing.
6) If one prays daily in such a shul, and is therefore accustomed to making the stipulation, we assume that the first counting (before nightfall) does not imply an absolute fulfillment of the mitzvah, and he may count again with a blessing even if he forgot to make the stipulation.
7) Counting before sunset does not fulfill the obligation, and one who counted before sunset must count again after nightfall with a blessing.
8) Even after the Sabbath has been accepted on Friday night, one may not count until nightfall, and an announcement should be made reminding the congregation to count the Omer after nightfall. Due to the prevalence of praying early (before nightfall) on Friday night, some insert a table of the Omer Count into their siddur or bencher, so that they will be reminded of the Omer Count when kiddush is recited.