I simply don't get all the hysteria over the party issue.
I was a single mum for many years myself prior to becoming a stepparent and obviously had party invites for my child which every so often clashed with their dad's contact time. It was no big deal, for me, my ex or our child as the simple rule was that we discussed it with my ex before accepting an invitation as a courtesy in case he'd planned anything which couldn't easily be undone. 9 times out of 10 there was no problem - we'd agree to "swap" a weekend, my child would go to the party and no-one would "lose out" .... my child had the party, I had my "me time" on another weekend, and my ex would see our child a weekend earlier or later. Our child was fine with this - even on the very rare occasions where their dad had booked non refundable tickets or committed to a family celebration because it was clear there was no game playing going on, no cries of "it's not fair, it's my weekend", and that the vast majority of the time a solution could be found which was acceptable to all. This was all done as soon as possible as well - no last minute surprises or disappointments. My ex lived about 40 miles away so it was clearly easier if I took our child to any parties.
My experience of parties with my partner's ex has however been horrendous and they have been just another opportunity for her to sh*t stir and play games. She lives over 100 miles away (she moved) so the distance compounds things but that's still no excuse for a total lack of basic courtesy and communication. If DP was "lucky" he'd be told, usually at the 11th hour, that the kids weren't coming that weekend as they had a party. No regard for any plans he may have made, no regard for his disappointment - or indeed the kids', and her being bloody rude because there's no way on earth she'd have only found out about a party on Thursday night when he was due to collect them Friday evening. I say "lucky" though because at least he wouldn't have had a wasted journey or have been placed in an impossible position, because her preferred method of dealing with parties on "his" contact weekends was often to let him collect the kids, as normal, without any mention of a party at all .... then text him (so no opportunity to have a real time discussion as she'd refuse to answer the phone) say, on Sat night (after shops had shut) or on Sun morning and TELL him the kids had to come home early as they had a party at 1pm - here's the address, you'll have to collect them as I'm out, deal with it. Again, no regard for our plans, no regard for the fact the car might not have been available at that time if I was using it say, no regard for us having to buy presents and cards (we're on a very tight budget) and putting us in an impossible position because the children had been told their dad was taking them to the party and obviously had no idea that this was news to him! Repeatedly asking her to communicate with courtesy had no effect and this dropping us in it happened many times over the years. We've had to endure upset and tantrums from the children on some occasions where we really did have genuine other plans - like family get togethers - which DP didn't want to cancel - or couldn't cancel, because other people were relying on us and would otherwise have been let down. It was awful, and when they were younger I used to dread contact weekends as we could never tell when this sort of thing might crop up .... it got so bad we'd ask the kids if they knew about any parties (tactfully) but obviously you can't expect or depend upon small kids to remember times and dates.
In our case, parties were nothing to do with what the kids wanted - though of course if DP ever dared to object to how rude and unreasonable she was being that'd be thrown at him - how he was stopping his kids doing what they wanted, how he was selfish not to take them etc. No, they were all about her causing us maximum disruption and unexpected expense. Thing is, had she been reasonable and discussed this all in advance, DP would have reacted just like my ex used to and would have been happy to swap weekends to accommodate parties - there'd have been no question of him insisting upon "his" time to their detriment.
Overall, while I accept that most children love going to parties and that it's nice to try and enable this wherever possible I do also think it's important to raise children who appreciate that the world doesn't revolve around them alone and that it's basic courtesy to consider the effects of what you do on others before you do it. Hence me always saying to my child that before accepting an invite we'd need to speak to dad. In a similar vein, my child also knew that there were some rare occasions where I might have made plans which made a party impossible that particular time. DP's ex though was of the "why should the children miss out because their parents are split" school which was hypocritical bollocks because this was only ever rolled out to paint DP as "bad dad" if he ever objected to the way she went about the whole thing - not parties per se - and never applied this maxim to herself when she declined invites because she had something different she wanted to do with the kids. It had nothing to do with a splitn and everything to do with her game playing, causing disruption whenever she could and being spiteful.