Hey. would go to Holiday Park at sunday and visit Phantasialand at monday. Holiday Park is a really "unpopular" Park, which is only crowded in Holidays and nearly empty on sundays. I think it's because of, Europa Park isn't far away. Because of you don't have so much to do then in Phantasialand i would visit it on monday because its (at the moment) the most popular park in Europe. So everybody jumps to PHL on the free sunday and they have to work again on monday. It won't be empty at monday, but less more crowded than sunday.
P.S. Sorry for my bad english, i hope you understood, what i'm trying to say.
P.P.S Have one with two of the best Coaster (Taron and Geforce) in our country!
Thanks a lot, that's what we'll do then.