During the spring and summer months, many people have the custom to accept Shabbat and recite Arbit before sundown on Friday afternoon. The question arises as to whether a person who follows this practice may count Friday's counting of Sefirat Ha'omer after he recited Arbit. Suppose the individual completed Arbit some 30 minutes or so before sundown, and then he remembered that he did not count the Omer on Thursday night or during the day on Friday. On the one hand, we might contend that since the sun has not set, the day has not ended and he may therefore still count Friday's counting. On the other hand, he had already prayed the Friday night Arbit service and accepted the onset of Shabbat, effectively declaring that Friday had ended and Shabbat has already begun. Perhaps, then, he can no longer count Friday's counting.
The Zera Emet (work of responsa by Rabbi Yishmael Ha'kohen of Modona, Italy, 19th century) addressed this question and ruled that a person in such a case should count the Omer before sundown without reciting a Beracha. Then, that night, he may count as usual with a Beracha. He compares this case to the Halacha cited in the Shulhan Aruch from the Terumat Ha'deshen (work by Rabbi Yisrael Isserlin, 1390-1460) regarding a person who cannot remember whether or not he counted on one of the days of the Omer. The Shulhan Aruch rules that such a person may continue counting with a Beracha, despite the possibility that he had missed a day of counting. (If a person knows definitively that he missed a day of counting, he can no longer count with a Beracha.) The Zera Emet extends this ruling to the case described above. After a person recited Arbit before sundown on Friday, it is uncertain whether or not he may still count the Omer. Therefore, he should count without a Beracha, and then his situation is no different than that of a person who simply cannot remember whether or not he counted on a given day. As such, he may resume counting with a Beracha after dark on Friday night.
This is the view accepted by Hacham Ovadia Yosef, in his work Yabia Omer (vol. 4).
Summary: If a person recited Arbit before sundown on Friday, and then realized that he had not counted the Omer that day, he should count before sundown without reciting a Beracha. He may then resume counting with a Beracha, as usual, that night.
