Marietta Edgecombe squeals on the D.A. to Umbridge. Rather than let Harry get expelled, Dumbledore takes the fall instead and is removed from Hogwarts. Unwilling to “go quietly,” he instead makes a dramatic escape.
We know that this happened on a Monday in April, but with Easter Sunday being on April 7, that leaves only one Monday available: April 1. This date is not canon, but it is very close to the actual date.