One week to be divorced... and counting

[Ripe]

Hello. I am back from the land of the undead and I am in dire need for help and guidance.

STBXW (Miss Oportunity) and I are separated since September 2015.
According to our agreement (not yet approved by the court, but we are already abiding by it), each parent has the kids with them from Monday after school until the following Monday morning, when the parent who has the kids takes them to school. The other one picks them up at the end of the day and so on.
This has worked so far and each parent has proven to be flexible and reasonable.
For instance, one week ago it was my weekend with the kids but someone was visiting STBXW and she asked me if the kids could stay at her place from Friday to Saturday. I said yes.
And this weekend, in exchange, although it was her weekend with the kids they came to my place from Friday to Saturday.
I have to say I am not in favour of this kind of swapping or trading. I think it is better for the kids' stability to stick to the planned schedule.
I was going to tell Miss Oportunity I didn't mind for them to go to her place in order to meet her friends and that I didn't need a compensation day, but I didn't. And, in fact, I had a compensation day.
But now my week with the kids starts tomorrow and Miss Oportunity asked me to keep the kids tomorrow because Tuesday she will be flying away and is only coming back the following Tuesday, so she is missing one of her Mondays, when the kids will have to be with me.
I want to say no, I want to say this swappings cannot become a regular thing. If chances dictates that one has one or two less days with the kids than the regular schedule, so be it.
What do you separated parents with kids think?
Should I concede?
Should I allow to trade the Monday?
Am I not wanting to do this for the wrong reasons, because I know the reason she is flying? And being that reason the signing of our divorce papers, so that the divorce becomes final?