The ISO-8601 standard defines a calendar system based on weeks. It uses the week-based-year and week-of-week-based-year concepts to split up the passage of days instead of the standard year/month/day.

Note that the first week may start in the previous calendar year. Note also that the first few days of a calendar year may be in the week-based-year corresponding to the previous calendar year.

Defined as starting on Sunday and with a minimum of 1 day in the month. This week definition is in use in the US and other European countries.

This allows a number of week-based-years to be added to, or subtracted from, a date. The unit is equal to either 52 or 53 weeks. The estimated duration of a week-based-year is the same as that of a standard ISO year at 365.2425 Days.

The rules for addition add the number of week-based-years to the existing value for the week-based-year field retaining the week-of-week-based-year and day-of-week, unless the week number it too large for the target year. In that case, the week is set to the last week of the year with the same day-of-week.

This unit is an immutable and thread-safe singleton.

This will look up appropriate values from the provider of localization data. If the locale contains "fw" (First day of week) and/or "rg" (Region Override) Unicode extensions, returned instance will reflect the values specified with those extensions. If both "fw" and "rg" are specified, the value from the "fw" extension supersedes the implicit one from the "rg" extension.

For example, users who are interested in using an English locale, but want the first day of the week that corresponds with the ISO-8601 standard can call

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Locale enIsoLoc = Locale.forLanguageTag("en-u-fw-mon");
WeekFields.of(enIsoLoc).getFirstDayOfWeek(); // returns MONDAY

The first day-of-week defines the ISO DayOfWeek that is day 1 of the week. The minimal number of days in the first week defines how many days must be present in a month or year, starting from the first day-of-week, before the week is counted as the first week. A value of 1 will count the first day of the month or year as part of the first week, whereas a value of 7 will require the whole seven days to be in the new month or year.

WeekFields instances are singletons; for each unique combination of firstDayOfWeek and minimalDaysInFirstWeek the same instance will be returned.

The first day-of-week varies by culture. For example, the US uses Sunday, while France and the ISO-8601 standard use Monday. This method returns the first day using the standard DayOfWeek enum.

The number of days considered to define the first week of a month or year varies by culture. For example, the ISO-8601 requires 4 days (more than half a week) to be present before counting the first week.

This is similar to ChronoField.DAY_OF_WEEK but uses values for the day-of-week based on this WeekFields. The days are numbered from 1 to 7 where the first day-of-week is assigned the value 1.

For example, if the first day-of-week is Sunday, then that will have the value 1, with other days ranging from Monday as 2 to Saturday as 7.

In the resolving phase of parsing, a localized day-of-week will be converted to a standardized ChronoField day-of-week. The day-of-week must be in the valid range 1 to 7. Other fields in this class build dates using the standardized day-of-week.

This represents the concept of the count of weeks within the month where weeks start on a fixed day-of-week, such as Monday. This field is typically used with dayOfWeek().

Week one (1) is the week starting on the getFirstDayOfWeek() where there are at least getMinimalDaysInFirstWeek() days in the month. Thus, week one may start up to minDays days before the start of the month. If the first week starts after the start of the month then the period before is week zero (0).

For example:
- if the 1st day of the month is a Monday, week one starts on the 1st and there is no week zero
- if the 2nd day of the month is a Monday, week one starts on the 2nd and the 1st is in week zero
- if the 4th day of the month is a Monday, week one starts on the 4th and the 1st to 3rd is in week zero
- if the 5th day of the month is a Monday, week two starts on the 5th and the 1st to 4th is in week one

This field can be used with any calendar system.

In the resolving phase of parsing, a date can be created from a year, week-of-month, month-of-year and day-of-week.

In strict mode, all four fields are validated against their range of valid values. The week-of-month field is validated to ensure that the resulting month is the month requested.

In smart mode, all four fields are validated against their range of valid values. The week-of-month field is validated from 0 to 6, meaning that the resulting date can be in a different month to that specified.

In lenient mode, the year and day-of-week are validated against the range of valid values. The resulting date is calculated equivalent to the following four stage approach. First, create a date on the first day of the first week of January in the requested year. Then take the month-of-year, subtract one, and add the amount in months to the date. Then take the week-of-month, subtract one, and add the amount in weeks to the date. Finally, adjust to the correct day-of-week within the localized week.

This represents the concept of the count of weeks within the year where weeks start on a fixed day-of-week, such as Monday. This field is typically used with dayOfWeek().

Week one(1) is the week starting on the getFirstDayOfWeek() where there are at least getMinimalDaysInFirstWeek() days in the year. Thus, week one may start up to minDays days before the start of the year. If the first week starts after the start of the year then the period before is week zero (0).

For example:
- if the 1st day of the year is a Monday, week one starts on the 1st and there is no week zero
- if the 2nd day of the year is a Monday, week one starts on the 2nd and the 1st is in week zero
- if the 4th day of the year is a Monday, week one starts on the 4th and the 1st to 3rd is in week zero
- if the 5th day of the year is a Monday, week two starts on the 5th and the 1st to 4th is in week one

This field can be used with any calendar system.

In the resolving phase of parsing, a date can be created from a year, week-of-year and day-of-week.

In strict mode, all three fields are validated against their range of valid values. The week-of-year field is validated to ensure that the resulting year is the year requested.

In smart mode, all three fields are validated against their range of valid values. The week-of-year field is validated from 0 to 54, meaning that the resulting date can be in a different year to that specified.

In lenient mode, the year and day-of-week are validated against the range of valid values. The resulting date is calculated equivalent to the following three stage approach. First, create a date on the first day of the first week in the requested year. Then take the week-of-year, subtract one, and add the amount in weeks to the date. Finally, adjust to the correct day-of-week within the localized week.

This represents the concept of the count of weeks within the year where weeks start on a fixed day-of-week, such as Monday and each week belongs to exactly one year. This field is typically used with dayOfWeek() and weekBasedYear().

Week one(1) is the week starting on the getFirstDayOfWeek() where there are at least getMinimalDaysInFirstWeek() days in the year. If the first week starts after the start of the year then the period before is in the last week of the previous year.

For example:
- if the 1st day of the year is a Monday, week one starts on the 1st
- if the 2nd day of the year is a Monday, week one starts on the 2nd and the 1st is in the last week of the previous year
- if the 4th day of the year is a Monday, week one starts on the 4th and the 1st to 3rd is in the last week of the previous year
- if the 5th day of the year is a Monday, week two starts on the 5th and the 1st to 4th is in week one

This field can be used with any calendar system.

In the resolving phase of parsing, a date can be created from a week-based-year, week-of-year and day-of-week.

In strict mode, all three fields are validated against their range of valid values. The week-of-year field is validated to ensure that the resulting week-based-year is the week-based-year requested.

In smart mode, all three fields are validated against their range of valid values. The week-of-week-based-year field is validated from 1 to 53, meaning that the resulting date can be in the following week-based-year to that specified.

In lenient mode, the year and day-of-week are validated against the range of valid values. The resulting date is calculated equivalent to the following three stage approach. First, create a date on the first day of the first week in the requested week-based-year. Then take the week-of-week-based-year, subtract one, and add the amount in weeks to the date. Finally, adjust to the correct day-of-week within the localized week.

This represents the concept of the year where weeks start on a fixed day-of-week, such as Monday and each week belongs to exactly one year. This field is typically used with dayOfWeek() and weekOfWeekBasedYear().

Week one(1) is the week starting on the getFirstDayOfWeek() where there are at least getMinimalDaysInFirstWeek() days in the year. Thus, week one may start before the start of the year. If the first week starts after the start of the year then the period before is in the last week of the previous year.

This field can be used with any calendar system.

In the resolving phase of parsing, a date can be created from a week-based-year, week-of-year and day-of-week.

In strict mode, all three fields are validated against their range of valid values. The week-of-year field is validated to ensure that the resulting week-based-year is the week-based-year requested.

In smart mode, all three fields are validated against their range of valid values. The week-of-week-based-year field is validated from 1 to 53, meaning that the resulting date can be in the following week-based-year to that specified.

In lenient mode, the year and day-of-week are validated against the range of valid values. The resulting date is calculated equivalent to the following three stage approach. First, create a date on the first day of the first week in the requested week-based-year. Then take the week-of-week-based-year, subtract one, and add the amount in weeks to the date. Finally, adjust to the correct day-of-week within the localized week.

The comparison is based on the entire state of the rules, which is the first day-of-week and minimal days.