I'm not scaremongering here, but we're seeing a LOT of booking requests for accommodation over Xmas and New Years Eve coming back from our reservation partners saying "No rooms available".
This is particularly the case on Ko Samui on Ko Pha Ngan -- where we're seeing on some days up to 50% of requests being knocked back.
That's not to say the islands are full, as only a fraction of the places on either island take online bookings, with many relying on direct bookings or walk-in only. However, if you're the type of traveller that prefers a reservation in advance, or, have your heart set on a particular spot, then I'd suggest you look at firming up your reservations shortly.
Sawasdee is our main partner for the Gulf Islands -- you can see their coverage of both islands via the links below:
Ko Pha Ngan
Hope that helps you avoid sleeping on the restaurant floor!