Trouble when using the weekday function

[sarahuff]

Hello,
When using the weekday function, I got a message error about incompatible types:

Sub MIX_FillDays(howmanydays As Integer, ByVal monthnumber As Integer, ByVal yearnumber As Integer)
    Dim fullday As String
    Dim dayoftheweek As Integer
    Dim dayname As String
    Dim ran As String    
    I = 1
    Sheets("taks1").Select    
    Do Until I = howmanydays
        fullday = I & "/" & monthnumber & "/" & yearnumber
        dayoftheweek = Weekday(fullday) '*** error happens here! ***
        Call MIX_TranslateWeekDayName(dayoftheweek, dayname)
        ran = "I" & I + 3
        Range(ran).Value = dayname
        I = I + 1
    Loop
End Sub

What does this error means? How can I solve it? Thanks