Yes you are able to pre book speciality dinning starting at 45 days prior to the sailing, however if you are staying in a suite it would be available to pre book starting at 55 days prior to the sailing.
In general, you really don't need reservations for the specialty restaurants. Usually the peak time is between 7 & 8 PM. I have walked to the restaurant after 8 PM and found table without any problem.
You can also make reservations online before you sail. On NCL's website, log into your MyNCL account and select your vacation. You should see the option for shore excursions, dining & entertainment. The dining and entertainment options will be available 45 days (55 days if you are staying in a suite) before your sail date.
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You can prebook Specialty restaurant reservations and shore excursions by logging into "My NCL" within 45 days of departure. Also, it's where you print your eDocs (tickets) and luggage tags.
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