The Monthly Invoice Day field on the AP Recurring Invoice form, header Info tab.

This field is enabled once the Post Monthly check box is selected.

Specify the day of the month to use as the Invoice Date. When you post this invoice, the batch month and this day will become the invoice date. If this field is left blank, the Invoice Date entered when generating the batch will be used. May be overridden in AP Transaction Entry once the invoice has been added to an expense batch (AP Recurring Invoice Posting).