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first ten days of the lunar month
「一日」 has other readings with different meanings.
一日の月は細くて美しい。
The crescent moon at the start of the month is slender and beautiful.
旧暦の一日は新月にあたります。
The first of the lunar month corresponds to the new moon.
昔の人は一日の夜空を大切にしたそうだ。
People in the past apparently valued the night sky on the first of the month.