DS(11) does an activity once a month, it has a cost. He does it with two friends, one of whom he no longer goes to school with so it is when they hang out. Quite a bit of effort is required by the people who run it and they have a special session for the end of term. He also has to put some effort into it in between sessions, which he does with a little reminding but no resistance. This activity has a long waiting list so attendance once committed is expected.
Next week it is this month's session, but he has just been selected by his peers at school to take part in an inter-form sports tournament and really wants to do it. There are two games next week and one falls on the day of the pre-arranged activity. There is no requirement that you have to attend both sessions to take part so he could still do the sporting activity later in the week. I've also found out via his brother that he is actually a sub so not guaranteed to play.
He desperately wants to play and I am happy for him not to sign up to the first activity next term if he no longer wants to, but I think he should attend next week. He has also said he doesn't necessarily want to give up the first activity.
I think I have enabled voting.
YABU - I should let him miss the monthly activity.
YANBU - He should accept he is already committed.