23 October is a public holiday in Thailand. It is the memorial day of King Chulalongkorn. King Chulalongkorn also known as Rama V passed away on 23 October 1910.
King Rama V was the first Siamese king to have a complete western education. He is considered one of the greatest kings of Siam.
King Chulalongkorn is displayed on the backside of the 100 Baht bills. On the foreground he is pictured in navy uniform. The background symbolizes the abolition of slavery which he perhaps is best known for.