晦日
Misoka.
the last day of a month
晦日 means the last day of a month. It can be read みそか (misoka) or つごもり (tsugomori).
Why today: October 31 is literally October's 晦日.
Language note: 晦日 (みそか / misoka) means the last day of any month. 大晦日 (おおみそか / Ōmisoka) is specifically New Year's Eve, so the two should not be treated as synonyms in every context.
An old calendar word can make an ordinary month-end feel newly visible.
Practice:
今日は十月の晦日です。
Kyō wa jūgatsu no misoka desu.
Today is the last day of October.
Sources:
Kanjipedia — 晦日: https://www.kanjipedia.jp/kotoba/0000800000
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