一日

1. Origins, structure, and basic meaning

• Structure: 一日 (ついたち) is a date noun meaning the first day of a month under this reading.
• Basic usage: 来月一日に店が開く identifies the opening date next month, while 毎月一日は確認する日 identifies a recurring accounts-checking date each month.

2. Common pitfalls and similar expressions

• Calendar date rather than one-day duration
  • Under the reading ついたち, 一日 means the first calendar day of a month. 毎月一日 likewise means the first of every month, not a duration of one day.