The religious observation of Advent has shifted and changed through the years. But how did we get to a point where an Advent calendar is also a non-religious Christmas countdown with chocolates or other treats?
Research:

Allen, Scott. “A Brief History of Advent Calendars.” Mental Floss. Dec. 1, 2010. https://www.mentalfloss.com/article/26522/brief-history-advent-calendars
“A Look Back at the Advent of the Advent Calendar.” The Journal Times. (Racine, Wisconsin.) November 25, 2006. https://www.newspapers.com/image/342040471/?terms=advent%20history
Beck, Katherine. “The Sweet History of Chocolate in Advent Calendars.” Tasting Table. Nov. 3, 2022. https://www.tastingtable.com/1084507/the-sweet-history-of-chocolate-in-advent-calendars/
Bostrom, Kathleen Long. “Waiting for Christmas: A Story about the Advent Calendar.” Zonderkidz. 2006.
Britannica, The Editors of Encyclopaedia. "Advent". Encyclopedia Britannica, 27 Nov. 2023, https://www.britannica.com/topic/Advent
Butler, Alban. “The Moveable Feasts, Fasts, and Other Annual Observances of the Catholic Church.” John Morris. 1775. Accessed online: https://www.google.com/books/edition/The_Moveable_Feasts_Fasts_and_Other_Annu/xL94Kwv5JkYC?hl=en&gbpv=0
“Advent Calendars.” German Christmas Museum. https://www.weihnachtsmuseum.de/en/adventskalender
Johnson, Maxwell E. “Between Memory and Hope: Readings on the Lirturgical Year.” Liturgical Press. 2022.
“Largest Advent Calendar.” Guiness Book of World Records. https://www.guinnessworldrecords.com/world-records/largest-advent-calendar
“President Eisenhower’s Grandchildren.” Getty Images/ Bettmann Archive. 1954. https://www.gettyimages.com/detail/news-photo/president-eisenhowers-three-grandchildren-join-in-an-appeal-news-photo/1177519748
Tanner, Jeremy and NEXSTAR MEDIA WIRE. “How did Advent calendars become a beloved holiday tradition?” The Hill. Dec. 11, 2022. https://thehill.com/homenews/3763921-how-did-advent-calendars-become-a-beloved-holiday-tradition/
Treisman, Rachel. “Advent calendars, explained: Where they came from and why they're everywhere now.” NPR. Updated Nov. 6, 2023. https://www.npr.org/2022/12/11/1141855237/advent-calendar-history-evolution#:~:text=German%20publisher%20Gerhard%20Lang%20is,one%20each%20day%20of%20Advent.
“The story of the Advent calendar.” SELLMER ADVENTSKALENDER. https://sellmer-adventskalender.com/en-us/pages/history-of-the-advent-calendar
Holcomb, Justin. “What is Advent? The History, Meaning, and Traditions.” Christianity.com. Nov. 13, 2023. https://www.christianity.com/wiki/holidays/what-is-advent.html
Blakemore, Erin. “What is Advent – and why do we mark it with treat-filled calendars?” National Geographic. Nov. 29, 2021. https://www.nationalgeographic.com/culture/article/advent-is-a-season-of-candlelight-reflection-and-expectation
“Porsche advent calendar on sale for $1million includes yacht and watch.” Metro UK. Sept. 26, 2010. https://metro.co.uk/2010/09/26/porsche-advent-calendar-on-sale-for-1million-includes-yacht-and-watch-532456/
Shain, Susan. “Day 1: The Joy of Counting Down.” New York Times. Nov. 30, 2022. https://www.nytimes.com/2022/11/30/style/day-1-the-joy-of-counting-down.html
“What is Advent?” United States Conference of Catholic Bishops. https://www.usccb.org/prayer-worship/liturgical-year/advent
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