I am making a cake 5 days ahead of time using buttercream ( I think ) I need to box it and put in the fridge. What would you do freeze it or just put in the fridge?

I personally would freeze it. Loosely wrap the cake in plastic wrap, let it sit in the freezer overnite to harden, then wrap as tightly as possible without squishing the cake or destroying any decorations, and let sit in the freezer.

anyone else?

I'd have to agree - freeze it. Anyway you can leave it in the box but then securely wrap with plastic or put in a plastic bag and get all the air out?

I was unsure of this a couple of months ago. Now I freeze cakes (undecorated though) and pull them out when I need one now. My thanks to CCr's for this help!!

How well does filling hold up in a frozen cake?

I had a cake order for a buttercream iced sheet cake with a FBCT (Dora cake in my photos) that I HAD to freeze as I was going to be out of town. I placed it, fully decorated, in a cake box, then double wrapped it with freezer paper and taped it up good. The lady I made it for, took it out to thaw the night before she needed it and said it was perfect. It was frozen for a total of 4 days. HTH.

Also, from my understanding, refrigerating a cake can tend to dry it out, whereas freezing can make it even moister.