Most Advent calendars start on December 1. But the actual first day of the Advent season changes every year. In 2019, that day was December 1. In 2020, it is today November 29.
https://www.ibelieve.com/faith/advent-prayers-week-one-the-hope-of-advent.html
https://www.usccb.org/prayer-and-worship/liturgical-year-and-calendar/advent