Ex wants to change custody schedule last minute again

This is the third time in two months she's asking to swap weekends without proper notice. I get that stuff comes up but I had plans with the kids already - took time off work and everything. How do you guys handle this without being the bad guy? I don't want to be rigid but I also can't keep dropping everything. Our custody agreement says 30 days notice for changes. Should I just cite that or am I being unreasonable?