At 12.16pm on Sunday, the sun will be exactly above the capital, according to the National Astronomical Research Institute of Thailand (Narit).
In theory, then, it will be the hottest day as Bangkok will be fully exposed by the heat of the sun, it added in a statement on Monday.
Narit deputy director Saran Poshyachinda said hoped nature could come to the aid of Bangkokians, and prevent the city from being torched by the sun.
Rain, clouds and other factors could block some of the heat on that day, he said.
The position between the sun and other provinces will vary depending on their unique positions, Dr Saran added.
The Meteorological Department forecasts that April will be this year's hottest month, with maximum temperatures of about 42-43 Celsius.
The average temperature in Bangkok so far this month is 35C, it added.