"rereleases (available every December 18th [and December 24, 2011]; every user can adopt a few random pets from past years)"
"You can rest assured that the rereleases only happen on the famous December 18 during the advent calendar. ^^ It has always been that way ever since 2008 (with the exception of the additional rerelease on December 24, 2011, but it happened because of a miscalculation, and it still happened during the advent calendar)."