Common year starting on Friday has only one Friday the 13th; The only Friday the 13th in this common year occurs in August. Leap years starting on Thursday share this characteristic, but also have another one in February. This is the only year type where the "n"th "Doomsday" (this year Sunday) is not in ISO week "n"; it is in ISO week "n"-1. In the (currently used) Gregorian calendar, the fourteen types of year (seven common, seven leap) repeat in a 400-year cycle (20871 weeks). Forty-three common years per cycle or exactly 10.75% start on a Friday. The 28-year sub-cycle does only span across century
Friday the 13th strengthen the superstition about Tuesday. In addition, in Greek the name of the day is Triti ("Τρίτη") meaning the third (day of the week), adding weight to the superstition, since bad luck is said to "come in threes". Tuesday the 13th occurs in a month that begins on a Thursday. In Italian popular culture, Friday the 17th (and not the 13th) is considered a day of bad luck. The origin of this belief could be traced in the writing of number 17, in Roman numerals: XVII. By shuffling the digits of the number one can easily get the word VIXI