I assume you are young and have to go with family? I missed a 3 day convention in my 20s (in a period when i hated being a witless and wanted out) by choosing to drive myself there instead of going on the bus or with friend or family.
That's what i told my family, i never went. when quizzed over why they never saw me for 3 days I asked them where they were sitting and I just told them i was over the other side of the stadium (8 - 10,000 were there).
Otherwise just fake an illness, or just say you are not going at all or that you will go 1 day only and choose the least offensive one to deal with.
If you are under 18 and living at home then you probably will have to go and just make do with a book to read or something to do while there. Make up your mind though that soon you wont be doing this any more and life will get better.