Do reservations made daily or can they be made for each night upon boarding the ship

Edited to clarify subject line. -LP

That answer depends upon what class of cabin you have booked. Per NCL's new Freestyle 2.0 program of perks:
Inside and Oceanview, you can reserve a table that night.
Balcony and Mini-Suite, you can reserve a table tomorrow night and tonight.
Suite, you can reserve a table every night.

Does this apply to the specialty restaurants only, or to the main dining room, as well? I've seen conflicting answers on this one.

Also -- kccruise, I added to the title of your post to make it more specific to the topic at hand.

That's for Specialty Restaurants.
The Main Restaurants and the Buffet don't require reservations, never have.

Of course, if you're wishing to reserve a table, or several tables for a large group, you can reserve them at the Main Restaurants too, just to make sure you can all sit down together.

Usually groups can only reserve tables early or late, not during the ever most popular 7 pm time slot.