There is a Mitzvah to count the Omer for seven weeks beginning on the second night of Pesach. The Mitzvah is to count both weeks and days. One makes a Bracha each time they count. If a person forgot to count at night they can count during the day the next day without a Bracha and continue making the Bracha the following night. If a person omitted an entire day (night and day) they should continue counting, but without a Bracha.