I would document every visit. Was he on time, late, called ahead, just flaked out, reasons he gave etc. Just the facts. Love the google calendar idea too.
I would document every request he made regarding making up the days he missed. If it actually works out for the kids and you - than be flexible. If they have plans that can't be changed for some reason on the day he requests, or if for some reason you can't make that day work, than don't agree to that day and document your reasons why.
Personally I would communicate only by text or email so there is a log to back up your documentation. I wouldn't remind him either. Ignore his ranting on that subject with no response at all. He can type his rants till his fingers hurt. If he has a day in mind it either works or not and if it doesn't say 'No that day won't work because xyz is there another day?' If he badgers you I'd ignore and/or copy and paste that over and over. If there are really no days that work between now and his next visit than I guess he misses out and he can quack all he wants but you have your documentation to take to court if it comes to that.
Sorry for the ongoing stress.
