The filling I am using for a cake is a cookies and cream recipe, with heavy whipping cream, powdered sugar and vanilla. I was wondering if it would be okay to leave it out for about a day after I finish and decorate it? I am using fondant, so I don't know if I'm suppose to refrigerate that?

Are you using this recipe:

http://www.cakecentral.com/cake_recipe-2079-1-Cookies--Cream-Filling.html

If so, 1/4 cup of powdered sugar isn't enough to stabilize (control the water activity) in 2-1/2 cups of heavy cream - so you'll need to keep the filled cake under constant refrigeration until it's time to be served.

In fact, once filled your decorating time will be limited to 2 hrs. segments after which the cake must be thoroughly chilled before further decorating.

Most fondant is fine when refrigerated, just remember not to touch any beads of condensation that may appear when the cake is removed from the 'fridge. The spots will evaporate (disappear) on their own.

If you don't want to refrigerate the fondant, then switch to a b/c based cookies & cream filling or even the marshmallow fluff one:

http://www.cakecentral.com/cake_recipe-5963-3-Cookies-and-Cream-Buttercream.html

http://www.cakecentral.com/cake_recipe-5773-4-Oreo-Filling.html

HTH

P.S. If you can find Rich's Bettercreme, (either at Gordon's Food Service or Sam's Club bakery) you can substitute that for the heavy whipping cream... The resulting filling would then be shelf stable for 5 days (under normal conditions).