E.G.
if you have 50/50 custody, maybe you can make a calendar that shows the entire year on one page... color the days you get your kids one color, and their days another color... after a month or so, you could see if it's really 50/50. if they are getting the kids every weekend in June because they have fun activities planned, then you get them every weekend in July or August. just show them the calendar and tell them it has to actually be 50/50.
then you can also book up weekends you want in advance. and then tell them that hey - it was on the calendar, and they should pick a different weekend to do that activity... and maybe have to have specific picked weekends booked atleast 4-6 weeks in advance.
just out of curiosity - why is it important to you that their father gets the kids for father's day? if it's really important to him, that's one thing.. but if it is because you really want to spend mother's day with your own kids, and figure he would really like to spend father's day with them because that is how you feel.. well, i think I would plan a mother's day with your kids on father's day.