Tradition Who should it be important to?

I would think that it should be important to all involved. Right? Maybe I am wrong. Here is the situation. For the past 25 years my side of the family has celebrated Easter dinner on the Saturday before Easter. The reason for this is at one time family used to travel long distances and usually there is only a three day window to celebrate so we had Easter dinner on Saturday. Here is the problem. My brothers wife is planning my nieces birthday party on Easter Saturday. Typically we have dinner at 2:00 ish however the past 4 years it has been at 5:00. While on a conversation with my mom she casually says to me "sister in law is planning a party for niece on the Saturday before Easter at 5pm". This made me very un-happy. I basically said that was our family day and that she shouldn't do that. To calm the waters my mom said that she could have Easter Diner during the day. That will not work since my dad and hubs have to work. Mom then says well we just won't have it this year. EXCUSE ME! Bottom line I am not sure how this is going to work. You see I am a strong believer in tradition and I think that when you marry in to a family that you have to respect the tradition on both sides of the family, now if there were family traditions that happened on the same day I would understand the need to compromise. Now don't get me wrong birthdays are important and her birthday is not on Easter Saturday, so the ability to choose another day is there. Is my family Easter going to be cancelled. Probably. Am I really upset over this. Absolutely. Will I go to the party? Not likely. Do I love my niece? Yes. Is it her fault? No. So Tradition is important to who? I am confused, hurt and frustrated.