It is marked with remembrance ceremonies held across the country to commemorate King Mongkut the Great, the fourth monarch of Siam from the Chakri dynasty who is remembered for introducing Western innovations and modernizing the country.

Mongkut was born on October 18, 1804 to Prince Itsarasunthon (future King Rama II) and Princess Bunrot (future Queen Sri Suriyendra). At age 20, he became a Buddhist monk, following an old tradition that all Siamese men should spend some time in a monastery. That same year, King Rama II died, but Mongkut didn’t inherit the throne after his father’s death because the nobility chose his older half-brother born to a royal concubine as the next king.

To avoid political intrigues and prevent a succession crisis, Mongkut chose to remain in the monastery and devote his life to religion. During his time as a monk, he pursued religious reforms and founded a new monastic order, Thammayut (Dhammayuttika Nikaya). Mongkut also pursued a Western education, studying languages and astronomy with sailors and missionaries.

In 1851, King Rama III died. Since he didn’t name a successor and all his sons were born to concubines, Mongkut voluntarily renounced his monastic status and ascended the throne at age 47. Feeling the pressure of Western expansionism, he sought to establish friendly relations with Britain and France to protect his country from being forcefully colonized.

During his reign, Rama IV embraced Western innovations and initiated technological and cultural modernization of Siam. Realizing the importance of European education, he hired an English woman named Anna Leonowens to teach his wives and children (this story became the inspiration for the Broadway musical The King and I and several films, which were at some point banned in Thailand).

European-style education was also popularized by missionaries, who taught modern astronomy, geography, and other subjects. Mongkut also began to westernize the Siamese army and supported the publishing industry.

Despite being a polygamist, Mongkut actually contributed to the improvement of women’s rights in Siam. He released some of his concubines so that they could choose their own husbands, banned forced marriages, and made it illegal to sell one’s wife to pay off a debt.

King Rama IV ruled Thailand for more than 17 years. In August 1868, he and his eldest son Chulalongkorn got infected with malaria during an expedition to observe the solar eclipse at Wakor that Mongkut had predicted. The king died on October 1 and was succeeded by his son who made a full recovery.

From 2011 to 2013, King Mongkut Day was celebrated on October 18 to commemorate the king’s birthday. However, it was too close to Chulalongkorn Day, celebrated on October 23, so it was decided to move it to October 1 (King Mongkut’s death anniversary). King Rama IV Memorial Day is marked with a state ceremony. Although it is an official holiday, it is not a non-working day unless it falls during a weekend.