What to do with relatives?Should I say some apologies?

I was invited to the kind of an event by my aunt's family in L.A. All my relatives from all over the U.S will be there. I came fr other country to study and living in northern CA. Since I have to work, I told her I cannot come, but she desperately want me to, but I am still not going. I just realize she will probably be disappointed but the event is tomorrow and I am feeling extremely unease right now. I should tell you guys what the event is about, it is for my grandfather who died when I was in kindergarten. Tomorrow is the anniversary and they are going to says prayers for him. I was not thinking too much about it. Now I am feeling kind of guilty for not going. I just started driving, and I can go with one of my aunt who live in San Francisco. I am so indecisive and not ready, but do you guys think, they will really be disappointed or something? It is not that I don't want to but I can't get up and go right now. Thank you.