The month of April/May.

Tradition says that the Buddha’s birth, Enlightenment and death (parinibbāna),¹ took place on the full-moon day of Vesākha. The Vesākha-pūjā was always celebrated by the kings of Sri Lanka.³ The full-moon day of Vesākha was chosen for very solemn undertakings, such as the crowning of Devānampiyatissa,⁴ the laying of the Foundation Stone of the Mahā Thūpa,⁵ etc.

This event apparently occurred while the Buddha was spending his last Rains Retreat at Beḷuva alone with Ānanda as his attendant, which would make the date of his parinibbāna three months later in January (ed.)