开门大吉 (kāi mén dà jí) — Open the Door onto a Great Auspicious Reading · The Threshold Graded 大吉

The last two characters are older than the door. 大吉 is already in the 《周易》: the fourth line of 家人, the Family hexagram, reads 富家，大吉 — enrich the household, greatly auspicious — on a ladder of readings whose top rung is 元吉. Temples kept the ladder, sorting a hundred slips into 大吉, 上吉, 中吉, 中平 and 下下, and a Japanese shrine prints a similar one on its omikuji. The library's 吉 page tells that history in full; here it is enough that 大吉 names the top rung of the temple ladder.

What the first two characters add is the place the reading is taken. 开 is the verb for lifting a latch or parting a pair of leaves; 门 is the threshold those leaves belong to. Put together with 大吉, the phrase does not invent a classical sentence. It grew up on lintels, the way 出入平安 did — folk vocabulary people could hang over a door without needing a book to license it. Chinese idiom dictionaries that hunt for a 出处 tend to land on living speech rather than a pre-Qin line, and the gap is informative: these four characters assembled themselves out of a door and a grade everyone already knew.

Its nearest neighbours show where the weight falls. 开张大吉 and 开业大吉 are said for a business: the Taiwan Ministry of Education dictionary glosses 开张大吉 as the auspicious phrase for a shop beginning to trade, or reopening after a closure. 开门大吉 sets the grade on the door instead, and so travels wider. A shop says it on the first morning back after New Year, when the doors open for the year's trade; a household says it as the year's first door opens; and it also turns up over the front door of a new flat, a folk habit rather than a rule. The phrase follows the door wherever it opens, which is why it sits so naturally as a 横批.

Spring-couplet culture has always needed a short horizontal line above the two vertical strips. The 横批 is four characters wide on most doors, and its job is general rather than specific — the couplet underneath does the detailed wishing. 开门大吉 is one of the lines that fills that slot because it names the door's own first motion and then stops. 吉星高照 names what is overhead; 五福临门 names five arrivals; 出入平安 names traffic in both directions. This one names the opening graded.

The custom is older than the four-character form. 宗懔's sixth-century 《荆楚岁时记》 already has households at 立春 pasting up the two characters 宜春. Four centuries later, on the last New Year's Eve of Later Shu, 孟昶 rejected a courtier's draft and wrote his own pair on peach boards — 新年纳余庆，喜节号长春 — the couplet later histories treat as the earliest recorded spring couplet. The boards were hung at the sleeping-chamber doors. Door, grade, and the turning of the year have been one gesture for a very long time; 开门大吉 is the compressed folk form of that gesture.