System.Globalization Namespace
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
