What should i do?please help!?

Ok so heres the story...i hung out with my best friend on friday and my mom told her that she could come over sunday or monday. So on saturday i went to my friends house for a slumber party and her mom asked me if i wanted to stay sunday because she was going to make nachos and strawberry daquries(the kid ones). And i told her that i didnt know if i wanted to stay or not because of my friend. I want to stay so bad but what about best my friend? I dont want to lie to her or blow her off so what should i do?