Convert forum calendar or create a popup calendar that has hidden images that only reveal themselves on that day.
display options: 1. on the normal forum calendar
2. a direct HTML element on the page, put there by template or headers/footers. element must have class="advent_calendar" like: <div class="advent_calendar"></div>
3. a link placed on the page, put there by template or headers/footers. link will open a pop-up box with calendar. link must have class="advent_calendar_link" like <a class="advent_calendar_link">Advent Calendar</a>