All of the timelines I've seen are generally : DAY 1 bake DAY 2 decorate DAY 3 deliver. Silly question, but if I deliver the day before an event , the cake is eaten on DAY 4. Or should I bring everything forward so decorate & deliver same day so it can be eaten next day?

generally you will deliver on the day of the event - the day its eaten.

I'm assuming you are not a pro, are baking for a friend and have to drop cake off at venue the day before?

Then you will be baking and decorating the night before you drop it off....unless you have read the excellent thread on freezing the whole decorated cake in advance?

Just type that into the search bar at top of page.

Depends on the cake recipe and the icing and your climate.  Some cakes dry out quickly (sour cream cakes), some improve with age (ie mud cakes 3-4 days /fruit cakes 10 days).  A cake 100% sealed in icing will not dry out too quickly.  Are you icing in american buttercream, smbc or fondant?  AB crusts, SMBC does not, Fondant cakes take longer to dry out.

Regardless, delivery to the venue the day before should only be done if you totally trust the integrity of the venue to not accidentally harm your cake (bump it, knock it, move it to make room for something else, vacuum around it, store it in fridge next to the cut onions etc etc etc).  It is more ideal to plan delivery just prior to the event starting (ie 1-2 hours).