Saudi Arabian National day is always celebrated on 23 September. Known locally as Al-Yaom-ul-Watany, it marks 23 September 1932, when King Abdulaziz announced the unification of the country as a kingdom.
If the National Day falls on Saturday, Sunday will be a public holiday as compensation. If it falls on a Friday, Thursday before will be a holiday. If these changes co-incide with other holidays, and extra day of compensation will not be given.
Since pre-Islamic times, the Arabian peninsula had been occupied by nomadic tribes.
The Islamic prophet, Muhammad united these tribes to create a a single Islamic religious state. In the years following his death in 632, the territory under Muslim rule rapidly expanded across the middle east, from Spain in west to modern day Pakistan in east.
In the 16th century, the region first came under control of the Ottoman Empire and though various sultanates would be formed from time to time, it wasn't until the collapse of the Ottoman Empire at the end of World war I that modern Saudi Arabia emerged.
Ibn Saud (the son of Abdul Rahman bin Faisal who had been the last ruler of the 'Second Saudi State'), became the Sultan of Najd, the central region of Arabia in 1921.
After conquering the Hejaz (western Arabia ) in 1925, Ibn Saud was declared King of the Hejaz on 10 January 1926. In 1927, he changed his title to King of Nejd.
Despite being King of both regions, he ruled the two parts of his kingdom separately for the next five years. It wasn't until 1932, after a two year campaign to suppress and defeat former allies, that the kingdoms of the Hejaz and Nejd were renamed and unified as the Kingdom of Saudi Arabia.