- 1993-12-01: Amnat Charoen, Sa Kaeo, Nong Bua Lamphu
- 1982-09-27: Mukdahan
- 1977-08-28: Phayao
- 1972-03-01: Yasothon
- 1947-10-01: Kalasin
- 1946-05-10: Nakhon Nayok, Nonthaburi, Samut Prakan, Samut Sakhon
However, when looking at the districts, April 1st is by far the most popular date. Since 1950, there were 410 newly created districts, most of them originally created as minor districts. Of these, 59 were effective April 1st, 14% of all districts. Other popular dates are May 1st (29 districts), July 1st (28 districts), July 15 (25 districts) and April 30 (21 districts). Only 11 districts were created on October 1st, the beginning of the fiscal year.
I don't know if the preferences of April 1st has to do with the fact that this date was the beginning of the Thai year until 1940. That date is also the anniversary of the Ministry of Interior, which started operation on April 1st 1892 with Prince Damrong as the first minister.