The formal nights will be on your first and last sea days.
You can rent tuxedos at:
Just an FYI: A tuxedo is not required and about 5% of the men will be wearing one. This is based on last month's cruise on Liberty.
However, if you want to dress up, go for it!
Just don't be offended if the people at the next table are in Polo shirts and Dockers.