The Cabinet of Thailand has added an extra day off to the Songkran 2018 holiday, thus extending it to 5 days long!
The Thai New Year break was initially supposed to be from 13 to 16 April. However, since an extra day off on 12 April has been announced to allow for more time for the Thais to travel back home, this has extended the holiday to a total of 5 days.
For tips on Songkran, read 5 Tips for Tourists Going to Thailand for Songkran. Although the entry was from last year, I believe the tips are still relevant.
If you intend to take a train to Hatyai or Bangkok to celebrate Songkran 2018, read Train 46 Padang Besar to Bangkok and No More Butterworth-Bangkok International Express 36; Now Padang Besar-Bangkok Train 46.
Remember that the Songkran 2018 holiday in Thailand now officially falls on 12 to 16 April.