Only by coincidence. The spring period is a national mourning time for 24,000 Torah scholars, students of the great Rabbi Akiva, who died en masse during those weeks. The 33rd day is either the one day during that period that none died, or the day they stopped dying (in other words, they only died during the first 32 days at all).
The summer period is a national mourning time primarily over the destruction of the Holy Temple in Jerusalem, but also for other tragic events that took place on the bookend days of that period, the 17th of Tammuz and the 9th of Av.
Shinna Minna Ma:
Many people do, that’s true, but many others have the custom to not celebrate from the first of Iyar (the 16th day of that period) until the end, with day # 33 still being an exception. Most people, therefore, do try to avoid making big celebrations even after 33, and those who do make such celebrations generally do so in the last week of said period, which is a little less mourning-ful due to being so close to the holiday of Shavuot.