They are most probably pagan, but I guess there's a legend about how some saint got mad when he encountered a bunch of druids praying to trees, and cut one down and dragged it inside to show them that the trees were not gods. Deforestation >:(

Here we kind of celebrate everything regardless of personal beliefs, because holidays are fun, and most of the time there's good food involved.
Singapore sounds like it's much more relaxed about religion. Here there are "interfaith" observances in some places, but it's unusual for people to really celebrate non-commercial holidays outside of their belief system unless they're related to someone who does.