Songkran (water festival)
Songkran is a festival held in April, which is the Thai New Year according to the lunar calendar.
They threw water at each other as if it were disrespectful. Songkran has been designated as a public holiday since Thailand adopted the Gregorian calendar instead of the Buddhist calendar in 1941, making January 1st (Western calendar) New Year's Day. At this time, like New Year's in Japan, everyone goes back to their hometowns, so Bangkok's population density decreases a little.
Origin
Just as Japanese people sprinkle water on graves, they used to have a custom of pouring water on pagodas, Buddhist statues, and even the hands of monks and superiors to purify them and ask for their teachings. Before we knew it, it turned into a festival where water was thrown on everyone.