The Iowa State Fair runs for eleven days every August and is the cultural event of the year in Des Moines. It is one of the largest agricultural fairs in the country — more than a million people pass through — and it earns the visit even if you have no particular interest in livestock or deep-fried food. Both are abundantly available.
The fairgrounds sit on the east side of the city and are National Register listed; the grand concourse buildings date from the 1880s. Eat your way along the Grand Concourse (everything on a stick, by tradition), watch the FFA kids show their hogs in the Pioneer Livestock Pavilion, and check the schedule for the grandstand concerts. Outside fair week the fairgrounds host smaller events but the real experience requires August.