Covers both the solar (Gregorian) leap year — 366 days with a February 29 — and the lunar leap year, which inserts an intercalary month (闰月) roughly every three years to keep the lunar calendar aligned with the seasons.
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The character 闰 is 王 (king) inside 门 (gate) — it literally shows the king inside the gate on the 'extra' day. In imperial custom, on the leap day the emperor didn't hold audience and stayed inside.
