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February 29 — the rarest day
February 29 only exists in a leap year — once every four years. People born on this day officially celebrate their birthday only once every four years. There are about 5 million such people in the world.
Dates
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Why the year doesn't divide evenly by 365
Earth orbits the Sun in 365.2422 days. To compensate for the quarter day, every 4th year a day is added. The Gregorian calendar has accounted for this since 1582.
Science
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Black Tuesday — October 29, 1929
October 29, 1929 — Black Tuesday, the largest stock market crash in US history. This day marked the beginning of the Great Depression that gripped the world for a decade.
History