On December 6, 2018, U.S. Customs and Border Protection (CBP) published in Cargo Systems Messaging Service (CSMS) #18-000715, the Periodic Monthly Statement (PMS) dates for 2019.
CBP allows filers who have signed up for PMS to pay for shipments entered or released during the previous month by the fifteenth business day of the following month.
CBP provides additional information on PMS on their user guide here:
https://www.cbp.gov/document/guidance/periodic-monthly-statement
The CSMS may be found here along with the download for the PMS dates: