ten days, ten years, full period

旬 is composed of 勹 (bag) and 日 (sun, day). 勹 is a pictograph of an afterbirth, meaning to wrap or encompass, which extends to mean completeness or a full cycle. 日 represents a day or a year. Because ancient people counted on ten fingers, they regarded ten as the completion and full cycle of numbers; thus, 旬 came to represent a period of ten days or ten years.