I am making a cake for a wedding on a Saturday in June. I am making a 3 tier cake, probably with fondant although possibly with buttercream icing. I was wondering how soon before the wedding I can start the cake with it still being fresh. I was hoping to make the buttercream earlier and freeze it, bake the cakes on Thursday and assemble and decorate the cake on Friday. Would this still be ok for the cake to still be fresh on Saturday night? Should I have the completed cake in a fridge overnight on Friday night? Also, does anyone have any suggestions about what filling to put in the layers of cake incase I can't get overnight refrigeration? Thanks so much!

You can bake the cakes now and freeze.
For a Saturday delivery I pull the cakes on Thursday that have already been torted.
Fill freeze, wrap and weight for settling.
Friday, Crumb coat, ice and decorate. Put supports in and partially stack

Saturday deliver.

use the sleeved fillings that are shelf stable.

Buttercream lasts a long time. You shouldn't have to freeze it.
Cake has about six days of freshness ... naturally you'd want most of those days on your customer's end of the time line.

.........Cake has about six days of freshness .......

Lots of things contribute to freshness including how warm &/or humid it is. The warmer & humid the less time for the cake. Heat & humidity can also cause b'cream to spoil.
Definately bake and fz at anytime from here on. Make the b'cream and any decorations that might be added like flowers. B'cream keeps well in the frig for many weeks but flowers should be stored at room temp.