We are having a meal to celebrate parents in law golden wedding. We have to travel a fair distance to attend and sister in law also coming over a fair distance. I assumed meal would be on day of actual wedding anniversary but sister in law has said she wants to go out with friends that evening to a reunion of friends from years ago with some coming over from Australia instead and the meal will be the next night. Now the friends reunion is actually happening both nights but she wants to go on the anniversary night as more friends are attending. I am really angry at her selfishness, we re celebrating 50 years of her parents being married, thats a fairly big deal in my book, so I think she is being selfish buy insisting its not on the day of the wedding anniversary or am I just being petty?