a young marriage in trouble

[lnlyshp]

So here is my dilemma this evening. Our schedule for this week calls for W to have son tomorrow night. She just texts and explains her friend is getting married and her coworkers invited her to go out for girls night tomorrow. So she is asking if I will keep him. I am torn on what to do and how to handle it.

When I asked to switch days a couple of weeks ago so that I could get some errands taken care of, she got upset with me about not sticking with the schedule and claimed she had already made plans.

I'll be honest, I didn't have any definitive plans set for tomorrow. I was planning to go out with friends and celebrate the semesters end, but I don't imagine it would be a problem if it was pushed to Saturday. However, I almost want to say "no" on principle alone. I can't change the schedule, but she can? She asked nicely, said she would appreciate it and everything, but I did the same a couple of weeks ago and she got really mad at me. I don't think that's fair. I almost think it would be better for her (and for my goals) to miss out on it and stay home with her son.

I know that if I don't take him, she will get upset with me again and the cold shoulder will continue. If I do take him, I feel like I will be showing her that it's okay for her to break the schedule and to take advantage of my kindness and my love for my son. I feel like I lose either way.

Any advice?