In the (currently used) Gregorian calendar, the 15 types of years repeat in a 400-year cycle (20871 weeks)۔ Forty-three common years per cycle or exactly 10.75% start on a Sunday. The 28-year sub-cycle does only span across century years divisible by 400, e.g. 1600, 2000, and 2400.
In the now-obsolete Julian calendar, the 15 types of years repeat in a 28-year cycle (1461 weeks)۔ Each leap-year dominical letter occurs exactly once and every common letter thrice.
The final two digits of Julian years repeat after 700 years, i.e. 25 cycles.
When starting to count in 2001 for instance, every 6th, 17th and 23rd year of these Julian cycles is a common year that starts on a Sunday, i.e. ca. 10.71% of all years. They are always 6 or 11 years apart.
^ ابRobert van Gent (2005). "The Mathematics of the ISO 8601 Calendar". Utrecht University, Department of Mathematics. 24 دسمبر 2018 میں اصل سے آرکائیو شدہ. اخذ شدہ بتاریخ 25 فروری 2017.الوسيط |url-status= تم تجاهله (معاونت)
