If you set an investment date for the 29th, 30th or 31st of a month, the order will be queued on that day, or the last day of any subsequent month which contains fewer days.
For example, if you set your order to start on the 31st of March on a monthly cadence, you will see your orders being queued at:
31/03
30/04
31/05
30/06 etc.