System.Globalization Namespace

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Defines different rules for determining the first week of the year.

Member name	Description
FirstDay	Indicates that the first week of the year starts on the first day of the year and ends before the following designated first day of the week. The value is 0.
FirstFourDayWeek	Indicates that the first week of the year is the first week with four or more days before the designated first day of the week. The value is 2.
FirstFullWeek	Indicates that the first week of the year begins on the first occurrence of the designated first day of the week on or after the first day of the year. The value is 1.

These calendar week rules depend on the System.DayOfWeek value that is designated as the first day of the week. 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 method.

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

Date	FirstDay	FirstFullWeek	FirstFourDayWeek
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 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.

Date	FirstDay	FirstFullWeek	FirstFourDayWeek
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

Calendar Class

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