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first day of the month; the first of the month
「朔日」 has another reading with different meanings.
朔日に神社へ参った。
I visited the shrine on the first of the month.
毎月朔日にはお参りをしている。
I visit a shrine on the first of every month.
来月の朔日から新しい仕事が始まることになった。
It's been decided that my new job starts from the first of next month.