There will be some marching around and some speeches about leadership and great responsibilities. There might also be some wise words passed on to help the chaps in there careers. On imagines something about how properly to keep an iPod waterproof when at sea.
As to what to wear; not so long ago I was all alone in a hotel room and the television was broken so I read the entire newspaper including the fashion section. What I learnt was this:
Wearing capes is the new not wearing capes, and camel is the new not camel.
My advise would be a camel cape. Of course you will have to catch one first.
There will be a lot of standing around (or sitting down, if you're lucky) outside watching people in uniform stand still or march. There will probably be a band. This will probably be followed by a buffet. This will in turn probably be followed by a piss-up. You should have the chance to change before the piss-up. For the first event, if you have a uniform, wear it, with medals, if not, smart casual will be okay, but dress for the weather. For the piss-up, wear what you would wear for an informal event in the mess.