Are the themeparks closed on the end of dec?

I'm planning to go to japan on the 21st to 28th of december. I need to know if the themeparks are closed during december or when are their closing hours.

Just look it up on the park's website?

Most (all?) are open because it is a major holiday season. Some have special admission rules for New Year's eve. I also recommend to look up the specific theme park's details or indicate which specific park you are intending to visit.

Sorry, only now I noticed that you are inquiring about the period through December 28. Note that a large majority of tourist sites and shops follows regular opening hours at least through December 28. Only from December 29 do a lot of museums start to close.

But again, most theme parks remain open even over the New Year holidays. And virtually all before the actual start of the holidays.