Setting the Sefirah Time
The Shulchan Aruch (489:1) notes that if one didn’t count the Omer at the beginning of the night, then s/he can count Sefirah for the rest of the night. Question is, when is the beginning of the night and why is it better to count then?
There are 2 main reasons noted for the counting as early as possible. The first is the idea of Zrizin Makdimin L’mitzvos – that when does ANY Mitzva, one does so as early as he can. The other reason is Sefirah specific – the idea of Temimos – of completeness in the counting. In this regard, the goal of Sefirah is a complete count of 50 days and the early we start the day, the better off we are in making the Sefirah complete (See Mishna Berurah 489:4 that if it is not daybreak, it is still considered Temimos).
One of the big questions in our community concerns the proper timing of the beginning of the night. Generally we daven Maariv ideally at nightfall. However the time of the fall of night is somewhat disputed. Is the setting of the sun the onset of night (Shkiya)? Or is the time that the stars come out the definition of night (Tzeis HaKochavim) ? The time in between is known as Bein HaShmashos. Can one count Sefirah then (This is especially important when the lateness of the hour makes it uncomfortable and not counting in Shul might lead people to forget to count later. Can they count after Shkiyah?
Tosafos (Menachos 61a) maintains that one may count Sefirah after Shkiya since Sefirah today is Rabbinic in nature and when we have a doubt regarding a Rabbinic Mitzva, we tend to be lenient. This opinion is shared by the Rosh (Pesachim 10: 40) but Tosafos himself notes an opinion that disagrees with this idea. The Rambam clearly holds that Sefirah is a biblical Mitzva even today and therefore any doubt would require stringency and a later counting time.
The Ran (end of Pesachim ) questions whether Temimos requires each day to be complete or whether the entire unit of 50 days needs to be complete – rendering the question of when to count each day mute except for the first and last days.
Shulchan Aruch (O.C. 489:2) rules like Tosafos but notes that those who are Midakdek in Mitzvos wait until Tzeis to count. The Mishna Berurah (14-15) explains that one who counts in Bein HaShmashos fulfills the obligation and can even make the Beracha then but L’Chatchilah one should not pursue this option.
In regard to the possibility of counting after Plag HaMincha – which often happens when we take early Shabbos – from the Beis Yosef (489) it seems that this may have been the way things were done in the past – when night was declared after Plag for safety reasons. Rav Asher Weiss Shlita suggests that in a case of emergency – like in a retirement home where the residents cannot wait until nightfall to count, there may be a basis to allow for the early counting.
Rav Elyashiv ztl (Kovetz Teshuvos III:84) disagrees and maintains that even if one can take early Shabbos – daven maariv and say Shema early based on Rabbi Yehuda’s opinion, one may NOT be lenient for Sefirah with a Beracha. (I imagine he would maintain that since one can count the next day without a Beracha and fulfill the Mitzva that way. Rav Asher Weiss argues that to do so with regularity causes people to be lenient with their approach to Berachos in general.
Rav Wosner ztl (Shevet HaLevi IX:123) concurs with Rav Asher Weiss Shlita but only in cases of great need.
Thus, ideally one counts the omer after full nightfall (Tzeis) – even the most lenient interpretation of nightfall -- as per the Elizabeth Minhag. If one counts after Shkiyah, one fulfills the Mitzva but ideally one should not attempt to rely on the lenient positions in regard to counting after Plag.