Wednesday is the longest day of the year, the summer solstice in the northern hemisphere.
Suparerk Karuehanont, communication director of the National Astronomical Research Institute of Thailand, said that on Wednesday, June 21, the Sun would be at its northernmost point in the sky.
There would be 12 hours and 56 minutes between sunrise and sunset in Thailand.
Consequently, it would also be the shortest night of the year.
The day marks the start of summer in the northern hemisphere and winter in the southern hemisphere, he said.
On Wednesday the sun rose at 5.51am and would set at 6.47pm, Mr Suparerk said.