Been seperated from my ex for 8 years. DS is 12.
He's never been particularly reliable. He's supposed to pick our son up every other weekend at 6pm on Friday and drop him back 6pm on Sunday. This rarely happens. He works all sorts of strange hours and sometimes he picks son up at 10pm on the Friday or more often, not until the Saturday morning. I'm not normally informed of this until Friday evening, making it virtually impossible to arrange anything for myself or for DS on that Friday evening as I never know what is happening. I appreciate he doesn't know what time he might finish work or how long he will be stuck on the M25, but I still find it very unfair on us. More often that not, he doesn't want to pick DS up on Saturday morning as it means he has to drive a long way out of his way to come back and pick him up so he would rather DS sit up til 11pm and get picked up and then driven for another hour to his GF's. All this I can live with but it's quite annoying and not exactly ideal for DS.
What I am finding hard to deal with is when he lets DS / me down for the whole weekend on a Friday evening. He will just announce that he has to work and that he can't have DS this weekend. I'm not convinced that he only finds about work that day - I'm sure he has more notice than that. 
Anyway, a friend of mine wants to organise a "warhammer" evening and sleepover (DS is quite into this) and would love to go. She has left it up to me to let her know a date. DP has 4 children who we have every other weekend too so as you can imagine, I quite look forward to having a weekend "off" and DP and I don't have any other babysitters so we really only get time together when DS is with his Dad and DSC are with their Mum.
So, do I arrange the "warhammer" evening on "my" weekend and send DS off while I stay at home with DSC and then hope that ex might take DS the following weekend or should I just arrange it for ex's weekend and then I KNOW DP and I will get an evening to ourselves, but also take the risk that I am saying to ex that's it's OK to just let him down on his weekend, something that I keep trying to drum into him is not OK.