Save the date
The most famous American millennialist movement to actually pick a date for the end of the world, The Millerites, picked Oct. 22.
Forerunners of the Adventists, William Miller and co. picked that date in 1843 based on interpretive reading of the numbers of the book of Daniel.
The religious group most actively pushing the idea of end of the world in America today has also actually picked a date: Oct. 21.
The date, according to the group, can be figured from the story of Noah, 2011 being exactly 7,000 years after the flood, God having said "7 days" and a day meaning a thousand years.
They say May 21, 2011 will the rapture, and that's the date that's gotten a bit of attention on their billboards and in the media, but they also set the second date in the fall for destruction of the world by fire. ("On October 21st, 2011, God will completely destroy this creation and all of the people who never experienced the salvation of Jesus Christ along with it.")
The two groups have similar interpretive methods and, obviously, are fixed upon the same subject, but why do the two dates come out so close together? Ostensibly, they're using two different starting places from which to count, Daniel vs. Noah. For both groups, the year(s), 1843 and 2011, respectively, seem like the important part and the day is a little more incidental. As far as I can tell, the recent, WE CAN KNOW! group isn't Adventist and has no connection to Ellen White or that tradition, which might connect them to the date. The Jewish calendar is lunar, so I don't think a particular festival or ceremony would always fall on that day...
So, why the connection of Oct. 21/22?