The month of April May.
Tradition says that the Buddha's birth, Enlightenment and death, took place on the full moon day of Vesākha (E.g., J.i.; BuA.248; Mhv.iii.2).
The Vesākha-pūjā was always celebrated by the kings of Ceylon (See, e.g., Mhv.xxxii.35; xxxv.100; Cv.li.84.). The full moon day of Vesākha was chosen for very solemn undertakings, such as the crowning of Devānampiyatissa (Mhv.xi.42), the laying of the Foundation Stone of the Mahā Thūpa (Mhv.xxix.1), etc.