28.
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The calendar repeats every 28 years due to the cycle of leap years, which occur every 4 years. Therefore, the calendar was the same as 2009 in 1981, and will be the same again in 2037. This pattern is known as the Gregorian calendar cycle.
well this is not an invention but it happend in 1981! in 1981 the Space Shuttle is the first reusable spaceship.
10 goals against the Edmonton oilers 2008-2009 season after the trade deadline Untrue: March 19 1981 Buffalo 14 Toronto 4
Belize gain its independence on September 21'st, 1981...
The first London Marathon took place on March 29, 1981.