Regardless of how NCL advertises their "sales," prices never change. The "regular" price advertised is totally made up, so any "sale" is just marketing. Ignore the "limited time only" sales as they'll be back tomorrow, just broken down a different way. Occasionally there will be small deals such as a CruiseNext bonus of $250 or double Latitude points. You are going to save the most money by booking off the peak times. Below is an excerpt from the NCL "terms and conditions" that says it all.
Original rate prices quoted are offering prices and may not have resulted in, and savings shown from original prices may not be based on, actual sales. From time to time, we may use original to mean the price offered during a previous seasonal time period, and therefore such prices may not have been in effect during the past 90 days. Quoted cruise prices are subject to promotions, discounts, and sales based on many factors including but not limited to seasonality, timing, range or mix of offers at a particular time, demand, availability, and capacity management.