firstDayOfWeek

FirstDayOfWeek refers to the day on which a calendar week begins. It is a convention that varies by locale, culture, and software, and it can influence how weekly ranges are displayed, how week numbers are assigned, and how scheduling interfaces organize dates.

Across standards and regions, different days are used as the first day of the week. The ISO 8601 standard defines Monday as the first day, and it uses Monday-based week numbering where Week 1 is the week containing the year's first Thursday. This approach is widely used in international contexts and in many programming libraries. In contrast, several regions, especially in North America, designate Sunday as the first day of the week, which shifts week boundaries and week numbers accordingly. Some locales and applications allow intermediate conventions or custom settings that start weeks on Saturday or Sunday, depending on cultural or business needs.

In computing and software, firstDayOfWeek is exposed through locale data and configuration options. Examples include: Python’s datetime module, where weekday() returns 0 for Monday through 6 for Sunday; Java’s DayOfWeek enum, which follows ISO ordering with MONDAY as 1 and SUNDAY as 7; JavaScript’s Date.getDay(), which returns 0 for Sunday through 6 for Saturday; SQL databases with DATEFIRST or equivalent settings to specify the first day; and spreadsheet programs where functions like WEEKDAY can be configured to treat Sunday or Monday as the first day. Locale-aware frameworks (ICU/CLDR) commonly store a firstDayOfWeek attribute for localization.

The choice of first day impacts week numbering, fiscal or academic calendars, and user interfaces. While ISO 8601 is widely adopted for international interoperability, many systems honor local preferences or user settings, making firstDayOfWeek a contextual rather than universal constant.