A lot of public holidays are based on the interpretation of the
local religious authorities.
Thailand and Singapore usually celebrate vesak one month
apart.
Vesak in Indonesia and SIngapore are usually one day apart.
Last (2007) Hari Raya Puasa, there was a clash of interpretation
of the lunar interpretation, some factions of Islam chose to
celebrate it 1 day earlier than the govt’s declaration.
Chinese new year in Singapore and Indonesia sometimes fall on
different days (1 day apart)
