I made a royal icing tiara and have it on the top of my buttercream cake. Will it be ok sitting on top until the party tomorrow afternoon or will the buttercream break down the royal icing?

The RI will absorb the grease in the icing and it might break down overnight. I don't worry when they are thicker Ri pieces, but tiaras are too delicate to take the risk. Also if you are transporting the cake the chances of it breaking en route is greater if it's on the cake.

It's best to hold off and transport it seperately (in a box stuffed with paper towels or something else to protect it), then put it on after the cake is set up.

Thanks CeeTee! I will CAREFULLY take it off and package it for transport.