How can I format a date to a week format?

There is nothing built into Word that's as simple as Excel's WEEKNUM function.

You could modify one of the field codes in the Word Date Calculation Tutorial to convert the date to the day of the year, divide by 7, and add 1:

{ QUOTE { ASK  dt Date: } {SET yd{={dt \@ d}+INT(({dt \@ M}-0.986)*30.575)- IF({dt \@ M}>2,2-(MOD({dt \@ yy},4)=0)- (MOD({dt \@ yyyy},400)=0)+(MOD({dt \@ yy},100)=0),0)}} {=(yd/7)+1 \# 0} }

Alternatively, you could use a macro, with a Quick Access Toolbar button or a MACROBUTTON field to launch it. For example, if the date is in a cell of a Word table, this will change it:

Sub DateToWeekNumber()
    Dim rg As Range
    If Selection.Information(wdWithInTable) Then
        Set rg = Selection.Cells(1).Range
        rg.MoveEnd wdCharacter, -1  ' exclude cell marker
        If IsDate(rg.Text) Then
            rg.Text = "Week " & Format(rg.Text, "ww")
        End If
    Else
        MsgBox "Click on a date in a table cell.", , "Week Number"
    End If
End Sub

Jay,

I started toying with this earlier and got distracted.  Something isn't quite right.  Consider:

Sub ScratchMacro()
'A basic Word macro coded by Greg Maxey
Dim oDate As Date
oDate = "12/31/13"
MsgBox Format(oDate, "ww")

End Sub

which returns 53 :-(

The OPs original 12/13/13 returns 50 instead of his expected 49.  But I think maybe is should be 50.