My opinion: Unless something unusual happens, I'd be surprised if the schedule of a Memorial Day weekend flight is changed after March 31. Still possible! But not likely.
+1
I think they tend to "finalize" the schedule about two months out, and load schedule changes on a weekend towards the end of a month.
"Finalize" is in quotes because they tweak flights all the time, but the majority of the updates seem to be done around that time.