The mourning period during the counting of the omer (sefirat ha'omer) lasts for 33 days, but there are three ways to count them:

1. Second night of Passover until Lag ba'Omer (33rd day of counting the omer).
2. Rosh Chodesh Iyar until the third day before Shavuot (excludes Lag ba'Omer).
3. Second day of Iyar until Shavuot.

You may change your observance from year to year, if necessary.