You probably have the same letter carrier five days a week, and a different one on the sixth day, which will be a different day every week, Monday this week, Tuesday next week, Wednesday the week after that, and so forth.
A regular letter carrier delivers the same route every day, 5 days a week.
Mail is not delivered on Sundays, and each letter carrier also gets one day off a week, which makes a 5-day work week. This day off rotates–Monday this week, Tuesday the next week, Wednesday the week after that, and so forth.
Regular letter carriers bid on routes. Routes are assigned according to seniority. If you have enough seniority, when the route that includes your own house comes up vacant, you can bid on it and go home for a “nooner” every day. 
If you notice that you have a different letter carrier every day, this means that you live on one of the Routes From Hell that is carried by a combination of people. These are what’s called Auxiliary Routes, and they’re the leftover parts of other routes that don’t really fit in anywhere. The length of a letter carrier’s route is determined fairly arbitrarily by supervisory personnel who “walk the route” and time it, to see how long it ought to take. Sometimes there are chunks of the city that don’t really make up an entire route, so the mail for these places gets handed over to people called Part-Time Flexibles, who are all the newbies, the people who don’t have enough seniority yet to bid on a route and get it.
Auxiliary routes nearly always have the worst service in the city, because the mail is delivered by people who don’t “know the route”, so a larger proportion of mail gets brought back to the office. On a regular letter carrier’s route, he gets used to the fact that your Great-Aunt Martha can’t spell your name yet. A PTF doesn’t know this, so, shrugging, he brings the letter back to the office marked “Unknown at this address”.
The regular carrier just carries 8 hours worth of mail every day, unless he wants to sign up for the Overtime Desired list and get time-and-a-half for carrying the extra mail on his own route. The money’s good, so a lot of letter carriers do this. Even if all the mail on his route hasn’t been delivered, he can still knock off after 8 hours and bring the undelivered mail back to the Post Office. There are plenty of other people who ARE on the Overtime Desired list who would love to get paid time-and-a-half, or even double time, for delivering it.
On the regular letter carrier’s day off, there is a very special type of letter carrier called a T-6 (which is short for a City Carrier Technician Level 6), who is assigned 5 routes and who goes from route to route each day, delivering the mail for five different letter carriers on their days off.
The Better Half is a T-6, which is how I know all this. He’s sitting here working on the other computer, and I’m asking him all this. He wants to know who wants to know. I tell him, “It’s General Questions, the guy’s just curious. He’s never home, so he never sees his mailman, so he’s curious.”
He says, “Well, tell him that his letter carrier is curious about him, too. ‘The guy who’s never home’…”
He adds, “Every mailman in the United States has seen a customer naked at least once.”
[sub]…and yes, he frequently does come home for nooners… [/sub]