Weekends tend to be more busy all year on Koh Lipe as Thai and Malaysian tourists flock to the island for a weekend break. Transport and hotel reservations are highly recommended on friday, saturday and sunday, especially when there is a national holiday.
High season is from November until early May: reservations for transport and hotels is recommended.
Peak season is around 15 December until the end of February: most rooms and transport tickets are fully booked months in advance and arriving on Koh Lipe without a reservation can be a very stressful experience.
Low season, or green season as the locals call it, is from May to October. There are a lot less tourists and it isn't difficult to find a quiet spot on the beach. Although the island is now open all year with 2 boats running to and from Pak Bara every day, some of the hotels and other businesses still close down. It's a great time to go if you enjoy quiet time on the beach and there a great hotel deals to be had.