The amount of time after a tragedy when it becomes acceptable to make fun of it.
"Man, she went off like Mount St. Helens!" = Acceptable within the 21.5 year rule. Mount St. Helens
tragedy was in 1980, well over 21.5 years ago.
"Man, she freaked out like the people on the planes right before they slammed into the towers!" = Unacceptable within the 21.5 year rule. Will become acceptable on March 11, 2023.