Gregorian calendar In the (currently used) Gregorian calendar, alongside
Sunday,
Monday,
Wednesday or
Saturday, 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 only spans across century years divisible by 400, e.g. 1600, 2000, and 2400. For this kind of year, the ISO week 10 (which begins March 8) and all subsequent ISO weeks occur later than in all other years, and exactly one week later than
Leap years starting on Thursday. Also, the ISO weeks in January and February occur later than all other common years, but
leap years starting on Friday share this characteristic in January and February, until ISO week 8.
Julian calendar In the now-obsolete Julian calendar, the fourteen types of year (seven common, seven leap) repeat in a
28-year cycle (1461 weeks). This sequence occurs exactly once within a cycle, and every common letter thrice. As the Julian calendar repeats after 28 years that means it will also repeat after 700 years, i.e. 25 cycles. The year's position in the cycle is given by the formula ((year + 8) mod 28) + 1). Years 4, 15 and 26 of the cycle are common years beginning on Friday. 2017 is year 10 of the cycle. Approximately 10.71% of all years are common years beginning on Friday. == Holidays ==