Chulalongkorn Day is celebrated on October 23rd every year.
Also called Piyamaharaj Day in Thai, it is the day that King Chulalongkorn, or King Rama V, died. He is one of Thailand's important past kings. The most important thing he did is abolish slavery but he also did many other reforms in order to modernize the country.
King Rama V’s long reign of 42-years was a busy era of full-scale reform and the emergence of Thailand into the modern age. He learned the systems of Western governments, made friends with major powers such as Britain, France, America and Russia and sent the royal children to be educated in the West. It seems he was prepared to learn from the West while at the same time resisting domination by the major powers. Above all, he was the first Thai monarch who traveled widely to neighboring countries in Asia and to Europe twice.
For more information, see http://www.thailandlife.com/thai-culture/king-chulalongkorn-the-great.html