History - extract from Wikipedia
The decision to agree to celebrate Wesak as the Buddha’s birthday was formalized at the first Conference of the World Fellowship of Buddhists
(W.F.B.) held in Sri Lanka in 1950, although festivals at this time in the Buddhist world are a centuries-old tradition. The Resolution that was adopted at the World Conference reads as follows, "That this Conference of the World Fellowship of Buddhists, while recording its appreciation of the gracious act of His Majesty, the Maharaja of Nepal in making the full-moon day of Wesak a Public Holiday in Nepal, earnestly requests the Heads of Governments of all countries in which large or small number of Buddhists are to be found, to take steps to make the full-moon day in the month of May a Public Holiday in honour of the Buddha, who is universally acclaimed as one of the greatest benefactors of Humanity."
Hence on Wesak Day, Buddhists all over the world commemorate three great events: The birth, enlightenment and the passing away of Gautama Buddha. As Buddhism spread from India to all parts of the world, the teachings were readily assimilated with the cultures of the people who accepted the teachings. As a result, Buddhist art and culture took on a rich variety of forms with profound gentleness and kindness as the Buddha expressly forbade the use of force. The practice of Buddhism was adapted in many ways to suit the nature of the various cultures that accepted it. As a result of this, Wesak is celebrated in many different ways all over the world. But in essence many practices have become universal. This sacred day is purely a religious festival, and not a festive occasion. On this day all Buddhists are expected to reaffirm their faith in the Buddha Dhamma and to lead a noble religious life. It is a day for meditation and for radiating Loving-Kindness.