Thingyan is a Bhuddist festival celebrated over four days usually, but here only one day is emphasised. At the end it will be the New Year. Originally the dates were calculated by the traditional lunisolar calendar but nowadays the dates are fixed at 13 - 16 April. Sometimes this coincides with Easter. Somehow this tells us why the festivities are enjoyed so much. Water throwing is a major feature of the festival, hence the English name.
This photo is from several years ago and is from Myanmar. You can see it is a National Geographic piece.