Low season in Koh Lanta
During the low season (May - October inclusive), it is fair to say that Ko Lanta shuts down. This is not just to do with the rain. Unlike mainland Krabi and Phi Phi, rough sea conditions mean access is relatively harder. The passenger ferries stop running altogether (although there is still access via the car ferry); and all off-island excursions (island tours etc.) do not run. This limitation to the number of non-beach activities available means there is little to do if it raining - as it often is.
In addition, the monsoon winds are unkind to Koh Lanta, washing all sorts of debris up on to the beaches (natural and man-made), littering them with leaves, wood and rubbish. Waves can be big and hotels protect their gardens from the ocean spray with ugly green netting across the access to the sand.
Most of the non-locally-owned businesses close during this period, making choice of food and drink limited; in those establishments that are open, you risk being the only customer.
Having said all of this, if you are lucky with the weather, low season is in many ways the perfect time to visit Koh Lanta - prices are cheap (the hotels that stay open slash their prices by up to half of those in high season) and the beaches are completely deserted. If you are looking for a "getaway from it all" holiday in a place that will be very happy to see you, Koh Lanta during May to October would be ideal. And of course, it does not rain all the time, either - see our weather page for more information. Bring plenty of books and a well-stocked iPod and you're all set.