They will probably do it about 5pm or there abouts. Time enough for just about everyone that is going to be onboard and still enough time so that you can clean up a little before hitting the dining room.
The 'muster drill' is also known as the "Lifeboat Drill" and is required by SOLAS and everyone must attend. The explain the use of your lifejacket and let you know where you are supposed to report to should it be necessary to take to the lifeboats.
It is called that as a hold-over to the more traditional nautical terminology. You 'reported' to your 'muster stations' where they had a roll call to insure all were in attendance. Transfered over into the boat drills.