In my case the letter said the ceremony was at 9am. I got there 8:40am and waited till the doors opened shortly before 9am. They called everyone by their position (number in the oath letter) to check the documents and then go in to see the officer to: turn in PR and stamp landing document, then enter the oath room (same place I did the test). There were 60 people taking oath at the same time, so it took awhile till everyone was in their seats. That being done, someone came and explained what was going to happen: judge would enter, say a few things do the oath, call each person by name to get their certificate, sing O Canada and that's it. Right about 11am it was all done, I stayed around a few mins extra to take pictures with the Judge, talk with some people and such.