decameron, ten-day period, season (for specific products)
旬 centers on a fixed ten-day cycle, which is the traditional way to mark time in months. The idea of a season for specific products extends naturally from this: a period when something is at its best.
旬 combines 勹 (wrap) and 日 (sun/day), suggesting a wrapped bundle of days — a ten-day period. This is a well-established semantic compound.
The enclosure 勹 wraps around the sun 日, like bundling ten days into a neat package. That package is a ten-day period, and when a food is in its best ten-day window, it's in season.
For ジュン, imagine a ten-day period in June when strawberries are in season: June -> ジュン, and the sun 日 inside the wrap 勹 marks the days.
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ten-day period (in a month)
season (for fruit, seafood, etc.); best time of the year (for)
last third of a month; 21st to the last day of a month
middle ten days of a month; 11th to 20th day of a month
first third of a month; first ten days of a month
first 10 days of the month
ten-day report
ten days
Pentecost
Shavuot; Feast of Weeks; (Jewish) Pentecost
Lent
fish in season
over ten days
published every ten days
off-season
ten-day period
month and ten days; ten months; short time
period of ten days
hottest (e.g. fashion); freshest; in vogue
killer demon; demon who strives to destroy all goodness