Hi, I have decorated a few cakes before for friends and family, but I have been requested to do a wedding cake for my sister's wedding in June.

  The design she likes has a combination of fondant icing and buttercream rosettes.  I am wondering if I can have the cake decorated the day before and then keep it in the fridge overnight, and also if it will be able to be set up in the morning and left out for the day at their reception or if it should be taken out of the fridge just a few hours before their reception?  Any suggestions would be greatly appreciated! Thank you!

It depends on what kind of fillings you have.  If you are doing any custards or fresh fruit, then it should be refrigerated.  I always refrigerate mine at least a little while before taking them anywhere, just to firm them up a bit for the car ride.  For a June wedding, especially if it's outside, I would definitely refrigerate overnight and then leave it out the day of the wedding.
 

If there are no perishable fillings, then there's no need to refrigerate it at all. 

If there are perishable fillings, then the cake can be outside of refrigeration for a total of four hours.  That includes icing, decorating, transporting and display time.  Generally, perishable filings and wedding cake don't go together.

I have't read the other replies but you must realize there are many factors to consider. 

The type of cake, icing and fillings used will make a big difference.  The location the cake is set up in also.

Some of the cakes I have set up were in place the night before (1 or 2 at the request of the birde) but most I tried to deliver just a couple of hours before the scheduled wedding time.

It does help a bit to frig the cake overnight but most of mine weren't.  (Look thru my photo gallery.)  I remember one I set up in CA summer that they insisted be set up around 10 a.m. The location was the front yard of a ranch without a single form of shade. Found out the reception didn't start until late afternoon ! :(   It was a distant relative who told me about that some weeks after.  He said the cake held up well except it was a bit dry.  *No wonder!*  Sitting in the hot sun for some 4-6 hours!