Unfortunately, this is an example of what appears to be pretty much standard in divorce cases. People who have no problem deciding what to do with the kids one day become unable to agree on anything the day after the divorce is filed.
The first defense is to have a solid, detailed
parenting plan which spells everything out in detail - dates, times, etc.
Even that's not sufficient. For example, our plan is quite specific (mother gets daughter from 6 pm the day before Easter to 6 pm on Easter on even numbered years, father on odd years, etc.). Unfortunately, we forgot to include a list of which takes preference. Next year, I get her for spring break and her mother gets her for Easter - but Easter falls on Spring Break. WOuldn't be a big deal, except we wanted to take a Disney Cruise - which leaves on Easter, so my ex would miss her day. I still haven't asked her if we can trade another weekend for Easter, but will need to do so soon to make the reservations before the prices start going up.
Bottom line is that no matter how careful you are, you need a mechanism to resolve the conflicts. The best thing is to be able to discuss things and reach agreement. If that doesn't work, you might want to have a neutral third party (priest, pastor, counselor, etc) who will hear both sides and make a recommendation. Mediation is next and court is the worst solution (partly because of the cost, but also because it's so unpredictable).
I think in our case, I'm going to suggest that if we can't reach an agreement we flip a coin.