From NBC News:
It's been debated for months and months, but on Wednesday the United States Postal Service finally will announce it's not going to deliver first-class mail on Saturdays anymore. Read more
The postal service's announcement, planned for about 10 a.m. EST, is expected to say that packages, mail-order medicine, and express mail will continue to be delivered on Saturday, but not letters, bills, cards, and catalogs. Post offices which are now open on Saturdays will continue to be open on Saturdays.
If you rely on the Postal System for your financial transactions (paying bills by check, etc) this will affect you.
Edited @ 0946 to add: per my local Post Office, this change is effective Mon 8/5. Mail will continue to be delivered uninterrupted to Post Office boxes. However, there will be no street delivery of mail, catalogs, etc on Saturdays
. Street delivery of packages, mail-order medicine, and express mail will continue