Your code is back to front, it hides what you want to remain visible. Also, in your dropdown list, it says 'Moday' not 'Monday'.
Private Sub Worksheet_Change(ByVal Target As Range)
Dim dataRange As Range
Dim col As Range, row As Range
Set dataRange = Me.Range("C6:CA1000")
If Not Intersect(Target, Me.Range("B3")) Is Nothing And Target.Count = 1 Then
Application.ScreenUpdating = False
dataRange.Columns.Hidden = False
dataRange.Rows.Hidden = False
If Target.Value <> "ALL" Then
For Each col In dataRange.Columns
col.Hidden = Application.CountIf(col, Target.Value) = 0
Next
For Each row In dataRange.Rows
row.Hidden = Application.CountIf(row, Target.Value) = 0
Next
End If
Application.ScreenUpdating = True
End If
End Sub