Need a passport, pronto? Saturday's your day. That's when anyone can walk into local passport agencies without an appointment and submit an application.
The advantage is not neccesarily speed-- walk-ins will still have to wait the regular processing time (4-6 weeks) to receive their new passport -- or pay $60 for expedited service, which cuts the wait down to 2-3 weeks.
The real benefit is getting a passport official to review your documents, which will spare you from mistakes such as sending an invalid birth certificate or forgetting to sign the application. Such errors can hold up the process for weeks.
Find the passport office nearest you on the State Department's website, which also lists the photos, ID and other documents you'll need to bring with you.