King Bhumibol Adulyadej's Birthday, National Day, and Father's Day are observed in Thailand every December 5. This day commemorates the birth anniversary of King Bhumibol Adulyadej, also known as Rama IX, who was born on December 12, 1927, in Cambridge, Massachusetts. His birthday was declared a public holiday, and it serves as National Day and Father's Day in Thailand, reflecting his significant contributions to the country. King Bhumibol ruled for seven decades, becoming the world's longest-reigning head of state at the time of his death in 2016. The day is marked by various celebrations, including ceremonies and public events honoring the king's legacy and his impact on Thai society. Despite the ascension of his son, King Maha Vajiralongkorn, Bhumibol's birthday remains the national day, highlighting the enduring respect and admiration for his reign.