I went on a date with a calendar.

This joke hinges entirely on a delightful bit of wordplay, specifically a pun. The humor comes from the double meaning of the word "dates." In the setup, "date" refers to a romantic outing between two people. However, the punchline cleverly twists this, using "dates" to refer to the numbered days marked on a calendar. The unexpected shift in meaning is what makes us chuckle, as our brains quickly process both interpretations.

The humor is amplified by the sheer absurdity of the scenario. The idea of going on a romantic date with an inanimate object like a calendar is inherently silly and sets the stage for the wordplay to land effectively. We all understand what a "date" means in the context of romance, and we all use calendars to keep track of "dates" throughout the year. This everyday familiarity with both concepts makes the unexpected connection even funnier.

Ultimately, it’s a simple yet effective joke that relies on a clever linguistic trick. It takes two completely different meanings of the same word and brings them together in a surprising and amusing way, showcasing the playful side of language.