I'm traveling in December. I arrive at Helsinki airport (HEL) at 9pm and would like to get to Turku as soon as possible. What is the latest night bus available? Alternatively, how early can I take one the next morning?
According to the Matkahuolto.fi central bus schedule search, the last connection from Helsinki-Vantaa airport T2 to Turku leaves the airport at 22:50, arriving in Turku at 01:30. (There's also an earlier bus at 21:50 which you'll likely make if your flight is on time.) Both also serve T1 (leaving 10 minutes earlier) and involve a change of bus at Espoo's Ikea along the way, but the buses are run by the same company (Pohjolan Liikenne) as a guaranteed service, so this should be pretty painless.
Should you miss the 22:50, the next services are at 0:30 AM, 01:25 AM and 03:45 AM. So you're definitely not going to get stuck at the airport for the night!
There are also other options if you detour via central Helsinki, eg. transferring to a VR train at Helsinki Central (last departure 23:00, but arriving slightly earlier at 01:00) and the supercheap Onnibus service (last bus from central Helsinki at 22:45). But if it's your first time in Helsinki and/or you're tired after a long flight etc, the direct bus is the easiest option.