WASHINGTON — The Postal Service said Wednesday that its plans for five-day-a-week service on everything except packages is designed to stem financial losses.

What is the official start date?

The week of Aug. 5.

How much money will this supposedly save?

About $2 billion a year, according to the Postal Service, by reducing the workforce by at least 20,000 employees through reassignment and attrition. It would also significantly reduce overtime payments. The agency suffered a $15.9 billion loss in the past budget year.

Why is the agency losing so much money?

Most of the red ink comes from a 2006 law forcing it to pay $5.5 billion a year into future retiree health benefits, something no other agency does. Without that payment and related labor expenses, the Postal Service sustained an operating loss of $2.4 billion for the past fiscal year.

Why even continue bringing packages on Saturday?

Those deliveries have increased by 14 percent since 2010, officials say, while the delivery of letters and other mail has plummeted. Online purchases have increased package shipping, forcing the post office to adjust to customers’ new habits.

What else stays the same?

• Mail to P.O. boxes still would be delivered on Saturdays.

• Post offices now open on Saturdays would remain open.

Do my taxes pay for the Postal Service?

No, but it remains subject to oversight by Congress, which, since 1983, repeatedly has passed measures requiring six-day delivery.

So can Congress block this decision?

It could prohibit any delivery changes in the next spending measure. A Senate hearing on operations is scheduled for next week, and congressional aides said postal officials would be questioned about plans to end Saturday letter delivery.

How big of a deal is this?

It would be the biggest change since the post office ended twice-daily service in the 1950s.

Are more changes coming?

Probably. The service needs to find $20 billion in cost reductions and revenue increases to continue to operate, Postmaster General Patrick Donahoe said. Already, it has cut its workforce — one of the largest in the country — by 193,000 through attrition. It also has reduced costs by $15 billion by consolidating mail processing facilities, eliminating 21,000 delivery routes and reducing hours at 9,000 facilities across the country.

What do most Americans think?

A New York Times/CBS News poll last year found that about 7 in 10 Americans would favor the change as a way to help the post office deal with in debt. The Obama administration also is supportive.

What are lawmakers saying?

• It’s “common-sense reform,” said Darrell Issa, R-Calif., chairman of the House Oversight and Government Reform Committee.