If you've got an iPad with iOS 6.0 ( iOS 6.1 solves the problem ) there's a funny bug if you have an All day event/anniversary calendar entry on April 1st.

Open your calendar and choose month view. Immediatelly, if you've got an entry on Aprl 1st,  the iOS calendar closes.

To solve the problem either update to iOS 6.1 or open the calendar entry (April 1st entry ) through a different view than month view and delete the problematic calendar entry.