It depends on how old they are and what information they've already received about the scientific and cultural aspects of this holiday. Main thing is to keep it within their abilities to process and understand what is being explained.
I told mine what I thought was enough without the risk of them being overwhelmed.
Just pay attention to their reactions when you're explaining it and enjoy this time with them. If they're too young they'll understand with time. After all they have a zevist parent/zevist parents, right?