For the 7 day cruise:
If you rent a tux from the people in the brochure that DCL includes in your Cruise Docs, they will have it delivered to your stateroom cabin on the day you arrive.
The standard package comes with 2 shirts and one jacket. You can order it with a white and a black jacket as well.
When you disembark, you just leave it in the wardrobe closet and the stateroom host will return it for you.
They make it very convenient for you.
There really isn't a formal night on the 3 or 4 day cruises so they do not include the brochure with those cruise docs.