Hello,

I am making wedding cake for this Sunday, I need to deliver the cake on Saturday evening, it will then be driven to the venue fully assembled on Sunday. (6,8, 10 inch)

The top and bottom tier are covered in marshmallow fondant and sugar crystals.  The middle tier will be covered in buttercream rosettes (using 50/50 butter and shortening). 

My question is whether or not it will be ok sitting at room temperature overnight.  From what I understand I should not refrigerate marshmallow fondant, it won't fit in my refrigerator anyways.  ;). Is that correct.  I'm hoping it will be ok room temperature once I assemble it tomorrow.  Any advice will be awesome!  Thanks!

I refrigerate mm fondant without problem. You'll find two schools of people here, but I'm in the school that refrigerated everything, ESPECIALLY if transporting assembled. 

If you search the forum for "fondant refrigerator", you'll find several threads discussing just this  

Thank you for that information.  Will the rosettes be ok at room temperature for a day?

I did leave a cake covered with marshmallow fondant in the fridge before.  It was a little sticky and looked shiny when I took it out

Best thing to do is to do a test case. It really depends on humidity and temp. I'm thinking about doing a rosette cake, but I'm going to try it on a small one first and then leave it out over night. It's humid where I live and about 85F so I'm worried about them too. Doing 50/50 sounds like a good idea though.

I put mmf on cookies, took them out of the fridge and they were shiny and a little wet like shivana1040 said. They dried just fine (took a day in the humid air though).

I know it's really too late for this cake but I have left b'cream covered & decorated cakes UNrefrigerated at room temp sooooo many, many times when I had my bakeries.   And the recipe I used for b'cream had 1/2 butter / 1/2 Crisco.  Never had a problem.   It can develop a light crust but no problems there.   In fact, I do believe I posted a recipe on here many years back for b'crem for air dried flowers.