I hate when people cancel plans last minute

I had plans for today with my girlfriend and her sisters. We were going out to a festival. They cancelled and hour before because they changed plans. Its a Saturday, so I usually use it to go to the farm, work at the house and do some pending tasks I can't do during the week. Them cancelling messes up my schedule. Now my girlfriend wants us to go tomorrow instead, but I had already made up my mind…