旬

A ten-day period, typically used as a formal calendar unit to divide the lunar month into three segments:

• 上旬: The first ten days (upper decade).
• 中旬: The middle ten days (middle decade).
• 下旬: The final ten days (lower decade).

Contrast with 十日 (shí rì): while both refer to ten days, 十日 typically denotes a quantity or duration, whereas 旬 often denotes a position within the calendar or a specific rhythmic phase of the month.

In some early or loose contexts, the term may broaden from a strict ten-day count to mean a general "period of time" or a "season." If a literal "ten-day period" disrupts the logic of the passage, consider whether the author is referring to a specific temporal window or the completion of a cycle.