In Theravada Buddhism, which uses a calendar based on the year of Buddha's death, Wesak is celebrated on the day of Uposatha (usually in May or June of their calendar) when there is a full moon.
仏滅紀元を採用する上座部仏教では、満月の出るUposathaの日（たいてい仏滅紀元の5月か6月）にウェーサーカ祭を行う。