Q. If someone has already said mariv after the plag, and remembers that he hasn’t counted sefira the night before, can he still count for that day now after mariv?
A. Mishna Berura (489, 12-17 and in Shaar Hatziyun ibid. 17) rules that until sunset, even after reciting Maariv following Plag Hamincha, it is still considered as day and someone that counted the next day sefira would have to repeat the counting with a brocho, after nightfall (Tzeis Hakochovim).
Igrois Moishe (O.C. 4,99) determines that if one forgot to count sefira Thursday night, and remembers only Friday evening after having an early Kabolas Shabbos and Maariv, and it is still before sunset he can count then the Friday sefira, without a brocho and he will not loose the future brochos on the remaining days. (See also Nitey Gavriel Pesach 3, 36, 8 for other similar opinions).
Horav Shlomo Miller's Shlit'a opinion in our case is alike, and after praying an early Maariv you can still count the day's sefira, without a brocho, and then after proper nightfall begins, continue to count the next day's sefira with a brocho.
Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit"a
Important Numbers
