There are several different opinions about when the students of Rabbi Akiva died, and which days we mourn for them.
- The 7 days of Pesach
- 3 Days of Rosh Chodesh
- 7 Shabbatot
If you add them up, there are 17 days. If you take away 17 days from 49, you're left with 32 days during which the students died. That's why, according to the Maharil, we celebrate Lag Ba'Omer on the 33rd day, to commemorate that the students only died for 32 days, even if they may have died on Lag Ba'Omer itself.
However, there seems to be a problem with this calculation. One of the days of Shabbat would have also been one of the days of Pesach. If so, the students would have died on 33 of the days, not 32. If so, according to the Maharil, shouldn't we celebrate on the 34th day of the Omer, not the 33rd?
On further thought, I think I have more insight into the Maharil. The Talmud (Yevamot 62B) says that the students of R' Akiva died between Pesach and Shavuot. It does not say that they died during Sefirat Ha'Omer (although, according to the Maharil, this is what practically happened, since no one died the whole of Pesach). This is why the Maharil counts all 7 days of Pesach as days when no one died, even though it was part of the time of their dying.
This strengthens the question I have above, since one of the 7 days of Pesach must be Shabbat.