December is when the south coast comes fully back to life. The monsoon is over, the sky is reliably clear, the sea is calming toward its best swimming and whale watching conditions, and every guesthouse, cafe, and surf school on the coast is open and operating. It is also the most expensive and busiest month of the year, and the travellers who arrive without planning ahead for accommodation tend to regret it.
The weather in December is excellent across the south coast. Daytime temperatures run between 27 and 30 degrees Celsius. Rainfall is minimal and mostly brief tropical showers in the evening if they happen at all. The sea is calming through the month: early December can still have some residual swell from the monsoon season, but by the middle and end of the month the water along most south coast beaches is clean and settled. Weligama bay and Polhena near Matara are calm enough for comfortable swimming by mid-December. Whale watching from Mirissa is running through December, with blue whale sightings reported from November onward and the frequency picking up as the month progresses.
Christmas week and New Year are the most congested and most expensive period of the year on the south coast. Guesthouses that cost $30 to $40 USD per night in October can cost $80 to $120 USD for the same room over the Christmas to New Year period. Popular places inside Galle Fort and on the Weligama beachfront sell out weeks or months in advance. If you are travelling over Christmas, book accommodation at least six to eight weeks ahead. If you have not booked yet, look at the towns slightly off the main circuit: Ahangama, Dickwella, and Talalla will have more availability and better value than Mirissa or Hiriketiya at peak.
Surfing in December is very good. The swell is consistent and the wind is typically offshore in the mornings at most south coast breaks. Weligama is at its best for beginners and intermediate surfers in December. Ahangama and Hiriketiya have consistent waves and the surf schools are fully operational.
One thing December has that no other month offers is the atmosphere of the reopening season. Cafes and restaurants that were closed or running on skeleton staff during the monsoon come back fully. New menus, new faces, the whole coast feeling like it has just woken up. If you arrive in early December before the Christmas rush, you get that energy without the prices and crowds of the last two weeks of the month. That first two weeks of December window is genuinely one of the better times to be on the south coast.