admit-one really isn't kidding by the way - I've seen people show up in pyjamas, onesies, all sorts...
Preferably wear something suitable for public viewing and that is comfortable for you. Nobody expects you to be wearing anything formal though, jeans and a t-shirt or similar is pretty much the default. You can wear pyjamas or a onesie or something, although it will probably raise a few eyebrows. Likewise you can go in wearing a suit, but likewise would probably raise a few eyebrows (and roll a few eyes for people thinking you're trying a bit too hard!).