Several parts combine into one character.

旬 is a compound ideograph: 勹 (a wrapping / enclosing curve) + 日 (sun / day). The composite reads as "the wrapping of a sun-cycle" = ten days. Ancient East Asian calendars divided each month into three ten-day periods: 上旬 (first ten days), 中旬 (middle ten days), 下旬 (last ten days). The character preserves an entire pre-modern time-keeping framework that still survives in formal writing across CJK languages.

Korean reading "sun." 上旬 (sangsun, first ten days of a month), 中旬 (jungsun, middle ten days), 下旬 (hasun, last ten days), 初旬 (chosun, beginning of a month — synonym of 상순). Korean writing maintains this division actively in formal contexts: 6월 (June mid-month), (early next month). News articles and academic writing use 旬 vocabulary for time-precision.

Mandarin xún, 2nd tone. 旬 (xún, ten days), 上旬 (shàngxún), 中旬 (zhōngxún), 下旬 (xiàxún). Distinctively, Mandarin extends 旬 metaphorically to age-by-decade: 七旬老人 (qīxún lǎorén, "seventy-decade elderly" = a person in their 70s). Each 旬 = ten years in this idiomatic usage. So 八旬高龄 (bāxún gāolíng) = "eighty-decade elder." This decadal use of 旬 is unique to Mandarin among CJK languages.

Japanese on-reading ジュン (jun) covers calendar usage: 上旬 (jōjun, first ten days), 中旬 (chūjun), 下旬 (gejun). Japanese also has the alternate on-reading シュン (shun) carrying the most culinary-cultural meaning: 旬 (shun, "peak season / prime time" — applied especially to seasonal food). 旬の魚 (shun no sakana, "fish in season"), 旬の野菜 (shun no yasai, "vegetables at peak"), 旬の食材 (shun no shokuzai, "ingredients at their peak"). Japanese cuisine's deep reverence for seasonality treats 旬 as a sacred concept: eating asparagus in May, ayu (sweetfish) in summer, matsutake mushrooms in fall, fugu (blowfish) in winter. The slang extension applies to celebrities and trends: 旬のアイドル (shun no idoru, "idol at peak popularity"). Even popularity has its season.

Memory aid: wrapping a sun (日) cycle (勹) — ten days as one wrapped unit of time.