当天
dāng tiān , dàng tiān · HSK 6
the same day
📅 当 = at/that + 天 = day → on that very day
当 means 'at' or 'that (same)', and 天 means 'day' or 'sky'. Together they mark something happening on the same day being discussed. Tone shift: dāng tiān = on that day; dàng tiān = that very day (emphatic).
当 — small ⺌ + crown 冖 + hand 彐 (simplified from 當: 尚 + 田)
Originally written 當, combining 田 'field' with 尚 as a phonetic. The early sense was 'to match' or 'face' a field of equal value, extended to 'correspond to', 'ought to', and 'serve as'. The simplified form 当 streamlines the upper 尚 component.
天 — 一 (one/sky) above 大 (big person)
Originally a pictograph of a person (大) with an emphasized head, representing what is above — the sky or heavens. The top stroke evolved into 一, symbolizing the vast expanse overhead. From 'sky' it extended to mean 'day,' 'heaven,' and 'nature.'