Use the ShowWeeks property to show the weeks header. The weeks header displays the week number into the year. If the SingleSel is False, by clicking into the weeks header, the entire week is selected. The DisplayWeekNumberAs property specifies the way the control displays the week number. Use the ShowNonMonthDays property to specify whether the dates that are not part of the month are visible or hidden.

In [ISO8601], the week number is defined by: 

• weeks start on a monday

• week 1 of a given year is the one that includes the first Thursday of that year. (or, equivalently, week 1 is the week that includes 4 January.)