윤년
This is the only common year with three occurrences of Friday the 13th: those three in this common year occur in February, March, and November. Leap years starting on Sunday share this characteristic and with the exception of skipped leap years, a leap year that begins on a Sunday falls exactly three years either side of two consecutive common years starting on Thursday - for example 2012 between 2009 and 2015. From February until March in this type of year is also the shortest period (one month) that runs between two instances of Friday the 13th.