solar cycle

Webster's Dictionary (1913)

a period of 28 years, at the end of which time the days of the month return to the same days of the week. The dominical or Sunday letter follows the same order; hence the solar cycle is also called the cycle of the Sunday letter. In the Gregorian calendar the solar cycle is in general interrupted at the end of the century.

From the entry Cycle.

Webster’s Revised Unabridged Dictionary (1913), from GCIDE 0.54