It is traditionally believed that the Buddha was born, enlightened and passed away on the date(tithi) of Vesak. Now it does not seem to be very probable that all three major incidents of one's life would occur on the same date. What is the earliest source of this information about Vesak in the scripture?
Without looking too deeply, it seems that the word "visākhapuṇṇamadivase" (day of the full moon of Visākha) only appears in the commentaries. I don't think there is any canonical reference to that date; though I can't think offhand of any sutta that is precisely self-dated.