Advent calendars are used to count down the days until Christmas. Many include daily activity and prayer suggestions that can help us prepare spiritually for the birth of Jesus Christ. (Updated links for 2021)
The USCCB’s Advent Calendar for 2021 (printable)
Loyola Press’s children’s Advent calendars for children and adults

Virtual Advent calendar with various reflections and activities, hosted by the Irish Catholic Bishops’ Conference.