Ever deliver a 2 tier cake the day before it's event?  Any tips?  

Keep it in the rerfigerator away from humidity if it's a buttercream cake!  

Discourage them from doing it this way and have them sign off on something that basically says you wouldn't be a bit surprised if their cake is destroyed after you drop it off and you are not responsible.

never done it but have seen many disaster stories on here from people that have

ADoes the venue know you're bringing the cake early? Has the client made arrangements for the cake to be stored correctly? Have you made the client sign a contract stating that you are no longer responsible once it's delivered, no matter what? All you have to do is read the Cake Disasters section to see what kind of mess can easily be made by venue staff and guests when a cake is left unattended for such a long time, and the baker is almost always held responsible.

I wouldn't personally be comfortable with this and would probably worry myself sick all night and then stop by the venue the next day anyway to check on it. I trust no one with a cake. People have no idea and think "nobody will notice if I move it" and disaster ensues. Good luck with this and if it is a must I would write up a special contract and take a million pictures of the cake before I left it.

AHow come you need to do that?