Attending the Muster Drill is mandatory and rooms are checked if guests are missing. The ship will not leave the home port until after the drill, so if people are missing, and the rooms all have to be checked, it means the partying can't start. Everything shuts down during the drill. You learn what to do and where to go in an emergency situation, so that's why it's so important - could really be a matter of life or death.
Your sign and sail card will have your muster station on it, and the map on the back of your room door will show you where to go. Also, any crew member will be able to direct you if you are not sure.
You didn't mention which ship you will be on, but I hope you have a great time.