You can use Len(StrFile) > 0
in loop check statement !
Sub openMyfile()
Dim Source As String
Dim StrFile As String
'do not forget last backslash in source directory.
Source = "E:\Planning\03\"
StrFile = Dir(Source)
Do While Len(StrFile) > 0
Workbooks.Open Filename:=Source & StrFile
StrFile = Dir()
Loop
End Sub