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 the start of Easter holidays around April 10, that leaves only one Monday available: April 5. This date is not canon, but it is very close to the actual date.