The last day of the current month is cutoff for the following month.
This allows time to get the boxes organised and packed ready for shipment on the 5th.
If you have chosen the current months box, the cutoff is the 20th of the month. This is due to needing 5 business days to pack and send your order out. All orders after the 20th will be added to the next month box and shipment even if current month is chosen.