When is Wesak Day?

Why is Wesak/Vesak Day on the 1st of May in Malaysia, and 31st of May in Singapore?

This is according to the Malaysian Prime Minister's Department and Singapore's Ministry of Manpower, respectively.





Is this correct?

Related links:

• Wikipedia: About Wesak/Vesak Day
• Malaysian Prime Minister's Department: Malaysia Public Holidays 2007
• Singapore Ministry of Manpower: Singapore Public Holidays 2007