先日
せんじつ · senjitsu · JLPT N3 · noun
day
📅 先日 = before + day: a day that came before, so the other day
先 means ahead or previous, as in 先生 (one born before, a teacher). 日 means day. Together they mean a recent past day. It is more formal than この間 and is common in polite speech and emails.
先 — 𠂉 (person variant) on top + 儿 (legs) below
先 originally depicted a person with a foot or step shown above the legs 儿, suggesting someone walking ahead. From this image of going before others came the meanings 'first,' 'earlier,' and 'ahead.' The modern shape preserves the 'legs' radical underneath.
日 — pictograph of the sun
日 is a pictograph of the sun, originally drawn as a circle with a dot or stroke inside representing the sun's substance. Over time the round shape was squared off into the modern rectangular form. It means 'sun' and by extension 'day'.