掲示板 Forums - etymology of 晦日
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I know 晦日 is an expression coming from 三十日, to mean "last day of the month", or more commonly "new year eve", but why had the pronunciation of 三十日 changed from さんじゅうにち to みそか? 
And also, why is there 大晦日 ?
(And, when I type おみそか as kana input, 大晦日 doesn't show up as a choice - I have to first type something like おお, turn it into 大, then enter みそか・晦日 seperately ;_; )
みそ is an old form for 30. It can also be written as 卅 or 丗:
And か is a regular suffix for days, like 七日=なのか.
So, みそか is just the 30th day of the month.
大晦日 means the "great thirtieth day", referring to New Year's Eve.
"Great" in the sense of being the final one, similar to how 大 is used in:
大詰め = final scene, finale
大引け = final/closing price (in trading)
Ah, I understand now, thank you for explaining so concisely 
Top > 日本語を勉強しましょう / Let's study Japanese! > Anything About Japanese