Patron saints are chosen as special protectors or guardians over all areas of life.
Traditionally people see them as symbols of how to live a better life.
You can have patron saints of all sorts of things, like jobs, organisations and animals.
For example, St Francis of Assisi loved nature and wildlife so he was made the patron saint of animals.
But nations can have patron saints too. England, Ireland, Scotland and Wales each has their own national day named after their patron saint.
Saint David of Wales
St David's Day honours the patron saint of Wales, Dewi Sant.
It is celebrated every year on 1 March by the people of Wales, and others around the world.
On the day, many people choose to wear Welsh national symbols, like a daffodil or leek.
Children take part in traditional Welsh dances, sing Welsh folk songs and recite Welsh poems, and take part in school concerts or eisteddfodau.
Saint Patrick of Ireland
Saint Patrick is the patron saint of Ireland and is celebrated for bringing Christianity to the country.
Ireland celebrates St Patrick's Day on 17 March.
Saint Patrick's Day started as a religious feast to celebrate Saint Patrick's work, but it's grown to be an international festival of all things Irish.
People take part in parades and dancing, eat Irish food, and enjoy firework displays.
The day is also famous for people wearing shamrocks, dressing up as bearded Irish fairies called leprechauns, and wearing all green.
Saint George of England
England's patron saint is St George, who has become a symbol of the country.
St George's Day is England's national day and is celebrated on 23 April each year.
According to the legend, St George killed a dragon and saved a princess when he was a soldier in the Roman army.
That's why he has come to represent bravery.
He is respected not only by the English, but by Christians in Israel, Greece and Russia and also by Muslims.
Saint Andrew of Scotland
St Andrew is the patron saint of Scotland and he is celebrated on 30 November. This is called St Andrew's Day.
He is also the patron saint of Romania, Greece, Russia, Ukraine and Poland.
Some people might mark St Andrew's Day by going to a special type of party called a ceilidh - pronounced "kay-lee" - where they do Scottish country dancing.
Traditional food may be eaten, like cullen skink - which is a type of fish soup - or lamb.