They can and do eat whatever they wish to eat during Advent.
Fasting is not required during Advent.
The Gloria is not said at Mass during Advent and Lent.
You open the first window of the advent calendar on the first day of December.
because hyms hymns are prayers, and you pray a lot during advent.
The Roman Catholic church has a tradition of lighting Advent Candles during the four weeks of Advent.
you are confusing advent with lent. during advent there are no dietary restrictions , but during lent the faithful may not eat any meat on FRIDAYS and AshWendsday except fish. You also can't eat meat on Fridays throughout the year unless you live in Canada or the United States, where some other penitential act can be substituted .It can be a prayer such as a Rosary or Stations of the Cross or it could be the giving up of something, like watching TV.
Advent is a time of preparation for the coming of Christ at Christmas.
The primary thing that you have to do during Advent is to prepare for Christmas, you do this through confession, Mass, and prayer.
In the Traditional Roman Rite the Alleluia is used on Sundays of Advent along with the Gradual, but if the Mass from the Sunday is used during the week the Alleluia is omitted. It is not omitted during Advent in the Divine Office.
People first started celebrating advent at the birth of Jesus.
I am not sure what you mean by Advent Saint. There are hundreds of saints who have a feast day during Advent, including St. Nicholas of Myra. However, if any saint would be named as the Advent Saint it would be St. John the Baptist as the Gospels during Advent speak of him preparing the way for the Lord.
There are hundreds of saints who have feast days during Advent. It is just a coincidence.