On all my sailings it has taken place at sea during the crossing. Each day they anounce the time and place for each deck to go. It is quick and painless. A crew member scans your cruise card so that they can tell who has and hasn't attended.
Sun Princess (2 times), Mississippi Queen, Dawn Princess, Coral Princess (2 times), Regal Princess, Grand Princess (2 times), Diamond Princess, Maasdam, QM2 (15 times)