Thingyan Festival is Myanmar new year festival. It is held in the month of Tagu (middle of April), which is the first month of Myanmar Calendar. Thingyan has been celebrated since Tagaung period (850 – 483 BCE) and it became prominent in Bagan era (107 – 1298 CE).
The festival of Thingyan is enjoyable with pouring or throwing water on one another. It is taken as one that cleanses one and all of all dirt and grime of the old year and cools and clears the minds of the people for the new year.
Merry-making at Thingyan Festival is intermingled with noble and pure activities of doing meritorious religious deeds in accord with the teachings of Buddha Dhamma. It is a Buddhist festival celebrated over a period of four to five days, culminating in the New Year.
The New Year takes place at virtually the same time as the new year celebrations of many countries in South Asia like China (Tai People of Yunnan Province), Laos, Thailand, Cambodia, Nepal, Bangladesh, India, and Sri Lanka.
References: Wikipedia and various literature