In the (currently used) Gregorian calendar, alongside Monday, Wednesday, Friday 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 Sunday.
The 28-year sub-cycle only spans across century years divisible by 400, e.g. 1600, 2000, and 2400.
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.