It's a glitch that has to do with April 1 being the start of daylight savings time, and the iPad calendar app struggling to reconcile 'all day events' on a day whose length varies. If you're in the US, most of the country will have a 23 hour say, but not Arizona for example, whose day is 24 hours since they don't acknowledge daylight savings time.
And then there are other countries that eitehr ignore daylight savings time, or who start theirs at different weekends....and when that hour precisely vanishes.....
computers are stupid. The calendar can't reconcile that an hour simply goes missing so it responds by crashing the month (largely because it probably can't figure out exactly when March ends.