Songkran is not only in Thailand that celebrate but also in Cambodia, Laos, and Myanmar so it's hard to say who owns because of the similarity in culture.
Chaul Chnam Thmey in the Khmer language and Boun Pi Mai Lao in the Lao language a usually start celebrates on April 14th to 16th, but Thailand celebrates on April 13th to 15th .
In Myanmar, Thingyan in the Burmese language celebrates a long time for 5 days on April 13th to 17th.
There are activities similar Thailand such as