Which is the first day of the week? And which is week 1 of the year? (FAQ - Time)
The seven-day week, which nearly corresponds to a quarter of a lunar month, has been established for thousands of years as part of the calendar cycle. Different religions and cultures have their own conventions about the significance of the days of the week, and where in the week the cycle is said to begin. Even in modern times there are variations to accommodate specific needs, for example the BBC uses a week running from Saturday to Friday (the "Radio Times" week) for programme planning and their programme week numbering follows this convention.
However, it is clearly useful to adopt a common standard for use in transactions between organisations and in material intended for the general public. Such a standard was introduced by the International Organization for Standardization (ISO) in 1975. In ISO 8601 (First Edition 1988-06-15) we find:
- Annex A.3 .... For commercial purposes, i.e. accounting, planning and similar purposes for which a week number might be used, Monday has been found the most appropriate as the first day of the week.
- 3.17 week, calendar: A seven day period within a calendar year, starting on a Monday and identified by its ordinal number within the year; the first calendar week of the year is the one that includes the first Thursday of that year. In the Gregorian calendar this is equivalent to the week which includes 4 January.
- 5.2.3 .... Day of the week is represented by one decimal digit. Monday shall be identified as day  of any calendar week, and subsequent days of the same week shall be numbered in ascending sequence to Sunday (day ).
So, although the answer to the question 'which is the first day of the week?' depends on the context, the answer to the question 'which day of the week is day 1?' is clearly 'Monday'. Week 1 of the year is the Monday-Sunday week containing the first Thursday of the year.