Hi!

I have a egg free dairy free cake decorated in a crusting buttercream due on a Saturday morning. Typically I'd frost & decorate the night before with fondant decorations but I just found out I have banquet dinner that night before! I was thinking about frosting and decorating that Thursday night. The party is 10:00 am on Saturday. I know buttercream especially crusting works to seal in and preserve the moistness of the cake. What I'm wondering is how many days in advance is ok? Or should I just plan on a late night on Friday? (I work FT during the day so that's not an option). Any experience or advice on this? 

You would be fine to do it Thursday night.  The buttercream will keep the cake moist.  

Ditto...you'll be fine one day early.  The leftovers (if any) will last one day less....no big deal :)

The only drawback is that the cake will not be as fresh for the customer - especially if they have leftovers they might want to save for a few days :(       Having said that, I have kept cake in the frig (for family) as much as 10 days (got shoved to the back & forgotten)  and thought it was almost as fresh as it was in the 1st place.