Ex wants to change custody schedule last minute

She texted me this morning asking if she could swap weekends because her new boyfriend is taking her to the coast. I have plans with the kids already, nothing crazy but we promised them mini golf and pizza. This happens like once a month. I'm trying to be flexible because I want the kids to see their mom happy but this is getting ridiculous. Do I just shut it down or am I being too rigid? How do you guys handle these requests?