On National Day, schools, shops and homes are decorated with green flags, green balloons, green lights and big paper palm trees. Some children paint their faces green and white. Some draw small flags on their cheeks. There are parades through every town and giant fireworks shows in the big cities.
Special performances happen across the country. The Ardha sword dance is performed in town squares. Camel parades clip-clop through Riyadh and Jeddah. Falconers display their birds. Schools put on plays and concerts. Many families gather for a huge meal at home, often kabsa or mandi.
National Day is also a time when people celebrate the things that make up Saudi life - the deserts, the palm trees, the Red Sea, the mountains, the food, the calligraphy, the poetry. Children at school often work on big projects beforehand, like maps of Saudi regions, model camels and giant flag mosaics.
It is not the only national-themed holiday. A newer one called 'Founding Day' is held on 22 February, remembering the original founding of the first Saudi state back in 1727 - nearly 300 years ago. So there are now two big national celebrations in the calendar.