AITA for my mom denying me to go to dinner on Mother’s Day with my grandma!

(Ok some thing to know before I ask this. My mother has given my grandam a lot of time in the past due to her being 83 with dementia and wasn’t asked if it was ok for them to make dinner plans, also I’m 14 with divorced parent)

Today my mother found out that there was a dinner planed at the most expensive place around Us that my grandma invited me to for Mother’s Day. my dad, my aunt (lives in Texas and we rarely see her) and my grandma and me where planed to go but when my mother found out she was livid, talking about how it’s her time and how much time she has given to my grandma, and even though I was going to be with her all day and gone for 2 hour out to dinner we are the ass holes, she didn’t have anything planned for dinner that night and wasn’t planning on going out, she’s mad at me for wanting to go out with my grandma because she is also a mom technically and there wasn’t anything planned, and I’m mad at her because she is telling her friends and saying I’m in the wrong.

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Am I the ass hole or is my mother?

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update: my mother has seen the post and did get mad at me (she told me to post it) and now doesn’t want me to be over at all Sunday because I’m “choosing grandma over her” I’ve told her many time that I’m not she still gets to have breakfast and lunch with me and the rest of the day so on her 24 hour day only 2 hours go to my Grandmother (which is a anther Mother)