When I took the interview it was all in one day but as i understand it it's the tech/HR interview and group exercise.
The group exercise is a game that you play together - would probably ruin the point of it if I wnent into detail. Team work and problem solving are the order of the day.
The interview will vary massively depending on who you get - could be fairly technical, could be more scenario based.
Deferring wasn't a problem for me - I passed in the summer and deferred until March. Don't think it's always an option but just ask HR and they'll tell you. There is always a danger that the pay/conditions could change between now and then so be careful if you decide to defer.
Hope that helps