If one regularly prays Ma’ariv at a late hour all year round, it is best that he count after praying at his regular time. If he counts by himself at the beginning of the night, he is liable to make a mistake or forget altogether. Moreover, there is an advantage to performing the mitzva together with the congregation.

However, if one is preoccupied and cannot pray Ma’ariv with a minyan after tzeit, and he intends instead to pray by himself later on, he should preferably count the omer right after tzeit, in order to fulfill the mitzva as soon as possible. Furthermore, there is reason to suspect that he might forget to count after he prays Ma’ariv by himself.[5]

There is a dispute among the Rishonim whether one who forgets to count at night can rectify his error by counting during the day. The Torah teaches us that the time for sefirat ha-omer depends on the time of the omer harvest, as it says, “start to count seven weeks from when the sickle is first put to the standing grain” (Devarim 16:9-10). According to Behag, the halakha follows the opinion cited in Menaĥot 71a that if the harvester fails to reap the omer at night, he may, be-di’avad, reap the next day. Likewise, the same is true of sefirat ha-omer: if one forgets to count at night, he may, be-di’avad, count the next day. Rabbeinu Tam, however, maintains that the halakha follows the opinion stated in Megilla (20b, 21a) that the mitzva of harvesting the omer applies only at night. Therefore, one who forgets to count at night cannot fulfill his missed obligation by counting the next day. In practice, the accepted halakha is that one who forgets to count at night should count during the day without a berakha. On one hand, we give credence to the opinion that one may count during the day. However, we omit the berakha in order to avoid reciting a possible berakha le-vatala (sa 489:7). Each subsequent night, he may count with a berakha (Terumat Ha-deshen, mb 489:34).