Wut should i do??? don't know which to choose!!!!?

ok so 1 of my friends invited me to go with them to a family friend's house in Fire Island (we live in NYC btw) this weekend and another friend invited me to her going away party (she and i were closer last year but kinda drifted apart this year and she has a new set of friends but her and i are still friendly). I got the dates mixed up and thought her party was next weekend but it is actually this weekend. I told my friend yes at first (to the party) but then changed it to no because the Fire Island is kinda a bigger deal (they are taking me out of school early to make the ferry and it is for the whole weekend). Was this the right thing to do??? it isn't too late to change my answer to the party so wut should i dooo?