My dad's birthday is this weekend and we are going to make him a cake for Saturday. The cake is chocolate cake layered with a chocolate pudding filling. Will it be better if it is all assembled the day of the party, or is it best for it to be assembled a day or so before to let it set up and meld together?

It will taste better made one day ahead of the party.

I'd say it depends on how much time you have!  I usually have to bake at least a few days ahead of time so I freeze mine.  With pudding filling, you'll just need to be sure you have space in the refrigerator for it!  

I bake ahead of time, freeze, and then thaw and fill, and freeze again, for a much shorter amount of time. I don't know if pudding will freeze well, though. 

I think cake has NO flavor, the day it is baked, in comparison to 3 days later. 

I do as AnnaBakesCakes. They are moist and delicous. The fillings and the cake all come together. Putting them in the fridge, can make them dry. I learned this from Leah_s on this site. She gives great advice. hth

I bake normally a day before and wrap my cakes with clear wrap allowing small amount of air and space between the cake and the plastic wrap(do not wrap cake tight!!!) right out the oven! while they are still warm/hot. I let them cool on the counter  and when they are completely cool i place them in the fridge with plastic on them for the following day to decorate.  I place it in the freezer 30min before frosting to make it easy to crumb coat. I feel that by doing all this my cakes a super moist and flavorful.

I also need to make a cake for a celebration and wanted to find out if a fondant covered cake will stay good overnight to be eaten the next day,

It will have a butter cream and fresh cream filling.

I'll also be making fondant made MAC make-up for the decorations to put on the cake. Would these be likely to crack if kept in the fridge overnight?

please help!!
 

Yeah it's fine staying out for a day, even two.

If a cake is covered in buttercream it will do fine a night in the fridge...it won't dry.  No need to thaw frozen cakes before filling or frosting either.  I find crumb coating goes so much smoother when the cake is frozen.

 I do, however, make sure the cake is cooled to room temp before wrapping and freezing.  You can end up with soggy cake when it thaws...particularly if you don't unwrap it after taking it out of the freezer.  I speak from experience on this!