thinking about it...though not sure if I'm the best one to give advice.
I will say your response needs to be clearer. Like the last misunderstanding, it is hard to figure out what happened here.
In the future you both need to be much clearer and much more specific about what the expectations are.
During initial conversation..."so, you will have them for the whole weekend and I will make up for the time without them another time..." or something like that.
If Sunday is your day, it would easy for her to assume you still wanted it even with MIL in town...can you see your part in the communication breakdown.
Maybe,
"Dear W, it was my impression that you wanted the kids the whole weekend because your mom was in town. I truly thought I was accommodating your request. In the future, let's make sure to clarify exactly what days we are talking about when we change the schedule. There's obviously nothing more important to me than spending time with the kids and I made other plans, giving up precious time with them, because I thought you wanted it. Nothing to hide, just ask me and I will always clarify what my intentions are and what I am thinking.
