盛ん
さかん · sakan · JLPT N4 · na-adj
prosperous
🌸 盛 = heap up / peak; 盛ん = at its peak, in full swing
盛 is the same kanji as in 盛る (moru, to pile up) and 盛り (sakari, the peak or prime). 盛ん (sakan) is that peak as an adjective: business booming, a sport popular, activity at full strength. Sakari → sakan: the prime of something.
盛 — 成 (chéng, phonetic) + 皿 (vessel)
A phono-semantic compound: 皿 (a dish or vessel) gives the meaning, while 成 supplies the sound. The original sense was grain heaped into a ritual vessel as an offering, hence 'to fill' or 'contain' (chéng). From 'full to the brim' it extended to 'abundant, flourishing, prosperous' (shèng).