Introduce a formula next to Active Cell

Not quite sure I understand...This will add the date in Col B the number of entries from Active cell to last row in active cell Column
Try this...

Option Explicit


Sub add_date()
With ActiveCell
    .EntireColumn.Offset(0, 1).Insert
    .Offset(0, 1).Resize(Cells(Rows.Count, ActiveCell.Column).End(xlUp).Row - ActiveCell.Row).Formula = MonthName((Month(Date)) - 1)
End With
End Sub