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