https://www.ncl.com/specialty-dining-package/faq
WHERE CAN I PURCHASE A SPECIALTY DINING PACKAGE?
Online in our "Dining" section at ncl.com/myncl or through Norwegian Reservations at 1-866-234-7350. Once onboard you may also purchase it on the first two days of your cruise by calling or visiting any Restaurant Reservations Desk. For three and four-day cruises, this package may only be purchased on the first day of your cruise.