CHICAGO (CBS) — All the zip codes in Chicago start with 606. As a date, that would be June 6th.

On the northwest side, one of the zip codes is 60618.

That lines up with the date and year.

So the folks at the Daniel Doffyn Post Office are celebrating this quirk of the calendar with Zip Code Day.

A special postmark, stamped by hand  in the 60618 on Wednesday, complete with Mr. Zip, a cartoon character dating back to the 1960s.

“If we didn”t have no zip code there would be no system,” said Martin Allen Jones, who’s been delivering mail for almost 35 years. The past 30 years in the 60618 zip code.

“The people are friendly. After a while you cultivate a relationship,” he said.

A job now made easier by technology like hand-held scanners that help the carriers load their trucks for more efficient delivery.

“Rain, sleet, snow or hail we deliver,” said Yvonne Langston, who helps conduct this well-tuned orchestra. Their instruments include trucks, carts and lots of letter carriers.

“There are 66 routes in the station, from 2,500 to 4,000 pieces per route.  You can multiply that,” said Langston.

There are 42,000 zip codes across the country. The lowest?  00501 for the IRS in New York.

The easiest to remember? 12345 for General Electric, also in New York.

You can mark your calendars for the next zip code day in Chicago: 60619.

That’s on the South Side, next June 6th.