So my little sister, whom I am pretty close with (we work three feet from each other) is my MOH.
I picked my wedding date (May 21st) about two months ago. I liked that date the best, and will share my anniversary with a good friend. I picked the date for no other reason.
About two weeks after we picked the date, I get a call from my sister that her very very close friend, who she considers a sister, but is not blood related like we are is having her wedding on the same day.
She now is throwing the fact in the face at how disappointed she is that she is going to miss her friends wedding. I jokingly referred to her as being stubborn about something completely wedding unrelated, and she says "I don't think I'm the one who's being stubborn, about something completely stupid, about something she chose for no obvious reason."
I have full flexibility in changing my date, but I don't want to. I like the date. Her friend could only book her venue for that one day in May. Since the original confrotnation about the conflict she has brought it up multiple times even though she clearly stated that she would rather be with me on my wedding date, and she will just have to deal with it. When I told her I didn't want her to be grumpy on my big day, she said she would "put on a happy face." Becausely that's exactly what I am most looking forward too! Ugh!
Basically the whole thing has a damper on it now. We used to be so excited to plan together, now every time I talk about something wedding she seems depressed. I'm really not looking forward to spending my wedding day with my favorite person in the whole world beside me and having her disappointed.
What should I do?
For perspective, my Mom thinks I should ask her to stand down or agree to never mention it again.
p.s. sorry if this seems out of order and blubberish. I'm super bummed

I forgot to add the option to the poll: Change my wedding date and accomodate her to keep the peace. However, I already hand made, hand stamped and embossed all my Save the Dates based on May 21st!