You mean like Easter, which is the first Sunday after the first full moon on or after March 21? We Americans are so ridiculous with our complicated "last Monday of May".
I'm not criticising the timing of memorial day, I'm criticising the use of a moveable holiday as a release date. I've never seen a UK movie advertised with a release date of "Coming Easter", largely because it moves about and people would shrug their shoulders and say "No idea when that is coming out then." and would miss the big day.
(Unless they are parents in which case they'll have a vague idea due to school holidays.)