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.
USCCB’s adult Advent calendar (printable PDF file)
Loyola Press’s children’s Advent calendar (printable PDF file)

Virtual Advent calendar with reflections, from Loyola Press. (Can also be printed off.)

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