I have a part-time nanny, who works for another family I know for the other part of the week. It's not a nanny-share, we each employ her separately, I have the nanny for Mon - Wed and she has Thu and Fri. I just happen to know the other family as our DDs go to the same pre-school.
The other family like to take the nanny on holiday with them for a couple of weeks in the summer. They can only do this by swapping days with me, obviously (so they have her for 2 full weeks and then I have her for extra days in other weeks). In March, I gave them a list of dates that I could swap, and said that as time went by, this could change (i.e. if you don't choose early, then particular dates might not be available any more).
Mum has just come back to me today, and wants a week that I originally said was fine, but isn't any more. In fact, I have very little availability to swap any days any more, as I work on the nanny days, and it's too late for me to change things around without inconveniencing other people. Also, i have holiday clubs booked and plans to do things with DC and family/friends for a lot of the other days, so having our nanny on any of those days would be a waste of time now.
We had the same problem last year, and I ended up inconveniencing myself quite a bit, having to work at night to make up missed hours, and feeling annoyed with myself for letting it happen. That's why I gave them dates so early this year.
AIBU to say no swap? I feel really mean, and I know the other mum will think I'm being unreasonable, as the summer is so long, but it is just fully organised now!