if it genuinley is the only day he can see Bilby, then i would just celebrate with maybe a special dinner/breakfast the day before.
If he can see her on another day and is just being a stubborn prat then that would be different.
But if its court ordered, then you may have no choice.
However, you will need to make sure you have all of your holiday acessses sorted out, as you may find that you miss out on things like birthdays and xmas if you dont have a "joint care for special days" thing organised
GL hon, I hope you get it sorted and life gets a little less stressful for you. *hugs*