Actually I dont disagree with the Adminstration.
Lets say they had the event on a Sunday, they would either have to pay all the teachers, Janitors and more who would have to work (You dont expect them to work for free)
And now imagine there are 25 kids in a class, Maybe 1 parent would come during the week, but on a Sunday Both parents and 4 grandparents (Maybe Great-Grandparents-Siblings etc) would come per kid, it would be a logistical nightmare