Googled
http://www.when-is.com/vesak-2017.asp
Guess it right...
they really decide by the Religious leader who observed the moon's phases. (In this case full moon)
As difference contrary hv diff Buddhism.
The 2 mean sect are Mahayana and Theravada
Singapore follow the Mahayana (Thailand)
Indonesia following Theravada (sri Lanka)
So their calendars are slightly diff.
Just my small finding