No such maintenance is planned or anticipated in June, so I'm afraid that the post referenced above was made in error. However, as others have mentioned, there will be two days in drydock for a routine hull inspection next week, April 26-27, as indicated on the NCL website by the two 6-day cruises before and after those dates.
To provide some factual details, an issue with one of the seven generators on board was identified during routine checks around 10 am on Tuesday, April 16th. The issue was corrected as quickly as possible, and the voyage resumed as quickly as possible. The total time needed to correct the issue was just over 40 hours. It was an extremely dedicated effort by the engineers and outside technical specialists, with teams working in shifts 24 hours a day to minimize the impact to passengers.
The most recent snow between Hilo and Kona was four days ago on April 15th, as reported by multiple media outlets. Here is one such link.