There are two production schedules.
Priority Mail is delivered to the USPS the same day as long as it is received before 11:55 AM (Eastern). Otherwise it goes out the next day. However, Priority Mail received between noon Saturday and noon Sunday goes out on Monday.
First Class (including Certified and with any return receipts) is delivered to the USPS the next business day as long as it is received before 7:55 PM (Eastern). Orders submitted after 7:55 PM are printed and mailed two business days later. However, mail received between 7:55PM Friday and 7:55PM Saturday goes out on Monday.