Advent is the period of four Sundays and weeks before Christmas (or sometimes from the 1st December to Christmas Day!). Advent means 'Coming' in Latin. This is the coming of Jesus into the world. Christians use the four Sundays and weeks of Advent to prepare and remember the real meaning of Christmas.
There are three meanings of 'coming' that Christians describe in Advent. The first, and most thought of, happened about 2000 years ago when Jesus came into the world as a baby to live as a man and die for us. The second can happen now as Jesus wants to come into our lives now. And the third will happen in the future when Jesus comes back to the world as King and Judge, not a baby.
Advent Sunday can be from the 27th November (which it was in 2016) to the 3rd December (which it was in 2017)! Advent only starts on the 1st December when Christmas Day is on a Wednesday (which will happen in 2019)!
No one is really sure when Advent was first celebrated but it dates back to at least 567 when monks were ordered to fast during December leading up to Christmas.
Some people fast (don't eat anything) during advent to help them concentrate on preparing to celebrate Jesus's coming. In many Orthodox and Eastern Catholics Churches, Advent lasts for 40 days and starts on November 15th and is also called the Nativity Fast. (Advent also starts on November 15th in Celtic Christianity.)
In medieval and pre-medieval times, in parts of England, there was an early form of Nativity scenes called 'advent images' or a 'vessel cup'. They were a box, often with a glass lid that was covered with a white napkin, that contained two dolls representing Mary and the baby Jesus. The box was decorated with ribbons and flowers (and sometimes apples). They were carried around from door to door. It was thought to be very unlucky if you haven't seen a box before Christmas Eve! People paid the box carriers a halfpenny to see the box.
In the United Methodist Church, we celebrate the four Sundays of Advent representing “Hope”, “Peace”, “Love” and “Joy”.
Taken from https://www.whychristmas.com/customs/advent.shtml.