The "first day" depends on the context:
- For the week: According to the international standard ISO 8601, Monday is the first day of the week. However, in countries like the United States, Sunday is considered the first day of the week for cultural and religious reasons, especially in Christianity.
- For the seasons: The first day of fall (autumn) in 2025 is Monday, September 22, marked by the autumnal equinox.
- For the year: The first day of the Gregorian calendar year is January 1, known as New Year's Day.
If a specific type of "first day" is intended (week, season, year, or something else), please specify for a more focused answer.
