Cakes that are very detailed in their decorations are usually put together the day before because many of the decorations are made ahead of time. You will find that to be true especially with cakes that contain many, many flowers, which are sometimes made months in advance.
You will also find that some of the very tall cakes actually contain dummie tiers, which have been decorated to match the bottom tiers, which is actually made of cake.
We just finished a cake for a boxing champion that needed to be a replica of a boxing ring and contain a giant pair of boxing gloves and a championship belt – all of these decorations were done the week before so they could dry properly, but the actual cake was done the day before.
Hope this answers your question. Welcome to CakesDecor!