The Counting of the Omer
2. When the entire night passed without a person having counted, he should
count during the day (1), without reciting a blessing (2). One can then
continue to recite a blessing before counting on subsequent nights. If,
however, one forgot to count during the day as well, one should count on
the subsequent nights without a blessing (3).
A person who is unsure of whether or not he counted the omer on a
particular night, and he did not count during the following day, may still
recite a blessing on subsequent nights before counting.
(1) According to many authorities, although the correct time for counting
is at night, one still fulfills one's obligation by counting during the day
(Mishna Berura 489:33).
(2) Here we are taking into account the authorities who rule that counting
during the day is not a fulfillment of one's obligation, and hence the
blessing would be in vain.
(3) There are authorities who rule that each day is a separate mitzva, and
thus, even though one has missed a day, one must count for the rest of the
49 days. However, there are other authorities who rule that the mitzvah is
to count all 49 days, and if one has missed a day, one can no longer
fulfill one's obligation; according to this opinion, once one has missed a
day, if one recited a blessing on subsequent days, the blessing would be
considered in vain.
One who counted incorrectly on the previous day, is equivalent to one who
forgot to count altogether, and thus counts on subsequent nights without a
blessing (Ibid. 489:35).