(In chronological order of the start day.)
Option 1:
Mourn starting from the first day of Pesach, and continue till Lag Baomer. Total days: 33.
Option 2:
Mourn starting from the first day of the omer (2nd day of Pesach), and continue till the 34th day of the omer, skipping Lag Baomer. Total days: 33.
Option 3:
Mourn starting from the day after Pesach, and continue till Shavuot, skipping Lag Baomer, Shabbos, and the 3 days of Rosh Chodesh. Total days: 32 (33 in Israel).
Option 4:
Mourn starting from the day after Pesach, and continue till Shavuot, skipping Lag Baomer, Shabbos, and the 3 days before Shavuot (sheloshet yemei hagbalah). Total days: 32 (33 in Israel).
Option 5:
Mourn starting from the first day of Rosh Chodesh Iyar (30th of Nissan), and continue till the day before Shavuot, skipping Lag Baomer. Total days: 33.
Option 6:
Mourn starting from 1 Iyar, and continue till Shavuot, skipping Lag Baomer. Total days: 33.
(The pattern of 33 days is assuming Roch Chodech doesn't fall on Shabbos. I believe Lag Baomer can't fall on Shabbos.)