After having attained Enlightenment, the Buddha wandered from place to place teaching his discoveries to people who are accessible to instruction, helping large numbers of them achieve various levels of spiritual attainment. He sent his followers to spread Buddhism in capitals, cities and upcountry, until Buddhism was firmly established and widely spread.
The Buddha passed away on Tuesday, the Vesak full moon day in the year of the small snake under the two Sal trees in the Sala Grove of the Mallas in Kusinara, capital of the Malla State, (nowadays located in Kusinagara of Uttrarapradesa, India) at the age of eighty (around 2545 years ago).
The performance of the rituals on Visakha Bucha day had been continuously observed in Jambudavipa or India, the motherland of Buddhism, for a long time before Buddhism spread to Sri Lanka andThailand. There, the Visakha worship has been continually observed to the present day.
In Thailand, Visakha Bucha observance began during the Sukhuthai period (around 700 years ago), because of the close religious relations between Thailand and Sri Lanka. Sri Lankan monks came to propagate Buddhism in Thailand and were highly respected. Thai monks also went to study in Sri Lanka. It's believed that, those monks introduced this ceremony to the King and people at that time.