Yes, starting in 2013, in certain large cities /suburbs, the United States Postal Service does deliver packages on Sunday's for Amazon.com purchases (and sometimes non-Amazon packages).
Sunday delivery from the USPS is mostly for Amazon.com packages and those with Amazon Prime 2-Day delivery.
The Post Office has been expanding their Sunday delivery to many parts of the U.S.
If you see that your Amazon.com package arrived at your local post office on Saturday, it's possible you'll receive the package in your mailbox or porch on Sunday.