Timing depends on your college, but generally 7-11 I think. In my college and the colleges around me, the freps started banging pots with spoons as we were walking out. My college is close to the Cathedral so it was a short walk and we are gowned so we had to wear a gown. The dress code is formal (suit and tie for men and dresses for girls). You will be seated in the Cathedral and a bunch of people will give a speech, 2 colleges will be matriculated at the same time and one student from each college will have to sign the code of conduct. Then all students will move out and take a full college photo on Palace Green.
I think you can get more information on your college's freshers page/message them.