Missed Day of the Omer

The Aish Rabbi Replies

That depends. If you neglected to count the Omer in the evening, and did remember to count the following day, then the next evening you should continue the count as usual, with a blessing.

However, if you neglected to count the Omer for an entire day (i.e. missed both the evening and the following day), then you are still required to count until the end of 49 days - but WITHOUT a blessing.

The reason why one is not permitted to say the blessing is because the Torah writes "You shall count... seven COMPLETE weeks" (Leviticus 23:15). Therefore, if a person missed counting one entire day, it can no longer be considered complete.