No, the main dining room is available for you each night of your cruise. During formal night, the majority of guests will dress in more formal attire (i.e. sport coats, cocktail dresses, etc). Non-formal nights are more casual.
You have the 5:30 seating so if you choose to dine in the main dining room, just show up a few minutes before 5:30 and you will be seated at your table (dining room & table number will be printed on your Seapass card). If you choose not to dine in the main dining room, you can enjoy dinner at one of the other free locations (i.e. Sorrento's, Windjammer Marketplace, SeaPlex Dog House, etc) or one of the locations that charge an additional fee (i.e. Izumi, Chops Grille, Johnny Rockets, etc). You only need to eat dinner at 5:30 if you choose to dine in the main dining room. For example, if you dine in the Windjammer, you can show up anytime during their dinner hours.
I suggest trying the different restaurants to see which one you like best. Each can offer a different atmosphere and selections of foods. When we cruise with our kids, we tend to eat dinner in the Windjammer because it offers us more flexibility and a bigger selection of foods. When we cruise without our kids, we like to eat in the main dining room so we can sit back and relax more. The main buffet on the ship is the Windjammer and it is open for breakfast, lunch & dinner. If you want something quicker, you can grab pizza at Sorrento's or a hot dog at SeaPlex Dog House.