OK, making sure that your Form is bound to your data source (whether that be a Table or Query), add your Date field to the Form. In this example, let's say it is named "MyDate".
Then go to the Properties of this field, go to the "Event" tab, and go to the "After Update" property and add the following VBA code on that action (modify field name, as needed):
Code:
Private Sub MyDate_AfterUpdate()
' If a non-zero date is entered, change date to first day of the month
If Me.MyDate > 0 Then
Me.MyDate = DateSerial(Year(Me.MyDate), Month(Me.MyDate), 1)
End If
End Sub
So, it should automatically update any date they select back to the first day of that month. You can also enter "mmm-yy" in the Format property of this field, if you like.
Note, that the user can still type in the date manually. So you may want to add a note on the sheet instructing them to use the Date Picker instead of typing it in manually.