King Bhumibol Adulyadej, the world's longest reigning monarch, has rarely left the hospital where he has been staying since September 2009, aside from occasional official appearances.
But recently his health has shown signs of improving.
The 84-year-old monarch will leave Bangkok in the late afternoon and travel by car to Ayutthaya to pay homage to 16th-century-queen Suriyothai, famed for sacrificing her life to protect her husband, according to an official schedule.
He will also inspect irrigation activities, receive a gift of land from the family of Prime Minister Yingluck Shinawatra and watch folk dancing, before returning to hospital the same evening.