Clashing Dates

Why is it, that for weeks there`s nothing special on, yet as soon as I`ve accepted an invitation to something, I discover the date of the event that I really want to go and see, and of another that I`d like to go and see (yes some Flower Festivals!) are exactly the same weekend? One of them was particularly good last year. And being a working person there is only one day of the weekend that I can go, so it`s no good saying I can do both. The thing I`ve accepted the invitation for isn`t even that interesting as all the other folk that will be there are in the older generation – in their 80`s! I think they only asked me, because they asked my parent to it. Yet, how can I get out of it? I`m in a quandary over what to do.