A member of the System.Globalization.CalendarWeekRule enumeration is returned by the DateTimeFormatInfo.CalendarWeekRule property and is used by the culture's current calendar to determine the calendar week rule. The enumeration value is also used as a parameter to the Calendar.GetWeekOfYear(DateTime, CalendarWeekRule, DayOfWeek) method.

Calendar week rules depend on the DayOfWeek value that indicates the first day of the week in addition to depending on a System.Globalization.CalendarWeekRule value. The DateTimeFormatInfo.FirstDayOfWeek property provides the default value for a culture, but any DayOfWeek value can be specified as the first day of the week in the Calendar.GetWeekOfYear(DateTime, CalendarWeekRule, DayOfWeek) method.

The first week based on the CalendarWeekRule.FirstDay value can have one to seven days. The first week based on the CalendarWeekRule.FirstFullWeek value always has seven days. The first week based on the CalendarWeekRule.FirstFourDayWeek value can have four to seven days.

For example, in the Gregorian calendar, suppose that the first day of the year (January 1) falls on a Tuesday and the designated first day of the week is Sunday. Selecting CalendarWeekRule.FirstFullWeek defines the first Sunday (January 6) as the beginning of the first week of the year. The first five days of the year are considered part of the last week of the previous year. In contrast, selecting CalendarWeekRule.FirstFourDayWeek defines the first day of the year (January 1) as the beginning of the first week of the year because there are more than four days from January 1 to the day before the following Sunday.

Dec 31 Mon	Last week of the previous year	Last week of the previous year	Last week of the previous year
Jan 1 Tue	Week 1	Last week of the previous year	Week 1
Jan 2 Wed	Week 1	Last week of the previous year	Week 1
Jan 3 Thu	Week 1	Last week of the previous year	Week 1
Jan 4 Fri	Week 1	Last week of the previous year	Week 1
Jan 5 Sat	Week 1	Last week of the previous year	Week 1
Jan 6 Sun	Week 2	Week 1	Week 2
Jan 7 Mon	Week 2	Week 1	Week 2
Jan 8 Tue	Week 2	Week 1	Week 2
Jan 9 Wed	Week 2	Week 1	Week 2
Jan 10 Thu	Week 2	Week 1	Week 2
Jan 11 Fri	Week 2	Week 1	Week 2
Jan 12 Sat	Week 2	Week 1	Week 2

Suppose the first day of the year (January 1) falls on a Friday and the designated first day of the week is Sunday. Selecting CalendarWeekRule.FirstFourDayWeek defines the first Sunday (January 3) as the beginning of the first week of the year because there are fewer than four days from January 1 to the day before the following Sunday.

Dec 31 Thu	Last week of the previous year	Last week of the previous year	Last week of the previous year
Jan 1 Fri	Week 1	Last week of the previous year	Last week of the previous year
Jan 2 Sat	Week 1	Last week of the previous year	Last week of the previous year
Jan 3 Sun	Week 2	Week 1	Week 1
Jan 4 Mon	Week 2	Week 1	Week 1
Jan 5 Tue	Week 2	Week 1	Week 1
Jan 6 Wed	Week 2	Week 1	Week 1
Jan 7 Thu	Week 2	Week 1	Week 1
Jan 8 Fri	Week 2	Week 1	Week 1
Jan 9 Sat	Week 2	Week 1	Week 1

The following example illustrates how the System.Globalization.CalendarWeekRule and DayOfWeek values are used together to determine how weeks are assigned. In the Gregorian calendar, the first day of the year (January 1) in 2013 falls on a Tuesday. If the designated first day of the week is Sunday, the first Sunday (January 6) is the first day of the first week of the year, and Saturday (January 5) belongs to the fifty-third week of the previous year. Changing the calendar week rule to CalendarWeekRule.FirstFourDayWeek defines Tuesday (January 1) as the beginning of the first week of the year, because there are more than four days between Tuesday, January 1, and Sunday, January 6. Using this rule, January 5 belongs to the first week of the year. For 2010, a year in which January 1 falls on a Friday, applying the CalendarWeekRule.FirstFourDayWeek rule with DayOfWeek.Sunday as the first day of the week makes Sunday, January 3 the beginning of the first week of the year, because the first week in 2010 that has more than four days is January 3 through 9.

code reference: System.Globalization.CalendarWeekRule#1

Each System.Globalization.CultureInfo object supports a set of calendars. The CultureInfo.Calendar property returns the default calendar for the culture, and the CultureInfo.OptionalCalendars property returns an array containing all the calendars supported by the culture. To change the calendar used by a System.Globalization.CultureInfo, the application should set the DateTimeFormatInfo.Calendar property of CultureInfo.DateTimeFormat to a new System.Globalization.Calendar.

Namespace: System.Globalization
Assembly: mscorlib (in mscorlib.dll)
Assembly Versions: 1.0.5000.0, 2.0.0.0, 4.0.0.0