On formal night you will see all manor of dress from tuxes and long gowns to t-shirts and jeans.
I would say to follow dress codes just as long as you wear either a nice dress or pant suit is just fine if you dont like to dress up a lot or do like we do and go to the buffet that night. Just let your wait staff know your not coming on formal night so they wont wait for you.
Buffet is not a buffet in the evenings, it is sit down and a small but delcious menu. Thats where the waiters in training are. 6:30 to 8:30 is the time you can go.
It is casual and is open all but the first and last night of your cruise.
Formal night is for everyone at dinner. Many love to dress up and this is their time to shine, great for family photos too.
I can only think of one thing new cruisers arnt invioted to do and thats a get together for platnium castaway cay club members. Those are people who have done 10 or more cruises with DCL. They get an invitation left in their rooms and must show it to attend.
Kathleen