Hannibal has the typical Midwestern climate: long cold winters, hot humid summers, pleasant springs and autumns. May, June, September, and October are the most comfortable months and the best for walking the historic district and climbing Cardiff Hill.
The biggest event of the year is National Tom Sawyer Days, held over the Fourth of July weekend, with parades, a frog-jumping contest, and a fence-painting championship. Late September brings the Big River Steampunk Festival along Main Street. Avoid January and February unless you genuinely want to see an iced-up Mississippi — many attractions reduce their hours or close.