～回
～かい · ～kai · JLPT N5 · noun
happening
🔁 回 = a box inside a box: going round again, so one more kai = one more time
回 originally depicted a swirl turning back on itself, so it means to return or go around. Each lap around is one occurrence: 一回 (ikkai) once, 二回 (nikai) twice, 三回 (sankai) three times.
回 — enclosure 囗 + smaller 囗 inside
回 is a pictograph of a swirling whirlpool or coiled spiral, shown as nested squares in modern form. The original meaning was 'to revolve' or 'turn around,' which extended to 'return' and 'a time/occurrence.' It also appears as a phonetic in characters like 茴 and 廻.