The following code should help. It moves from sheet to sheet, based  on
what you enter  into the array, until it finds the value you are looking
for.

Dim i As Integer
Dim a As Integer
Dim k As Integer
Dim aSheet() As Variant

aSheet = Array("Sheet1", "Sheet2")

For k = 0 To UBound(aSheet())
Sheets(aSheet(k)).Select
Range("A1").Select
i = Cells(Rows.Count, "A").End(xlUp).Row
For a = 0 To i
If ActiveCell.Offset(a, 0).Value = "PONUMBER" Then
ActiveCell.Offset(a, 1).Value = "Date"
Exit Sub
End If
Next a
Next k