Why did the socks go to the party?

This joke is a prime example of a pun, a form of wordplay where a word or phrase is used to suggest two or more meanings, often for humorous effect. The punchline cleverly plays on the common phrase "party time" by substituting it with "pair-tyme." The humor clicks because socks are almost exclusively found and worn in pairs, making the idea of them wanting a "pair-tyme" a perfectly fitting and silly aspiration. It’s a linguistic trick that makes you appreciate the cleverness of the sound-alike words.

Socks have quite a long and storied history, evolving from simple animal skins tied around the ankles to the diverse array of fabrics and styles we have today. Throughout history, one constant has remained: their tendency to come in pairs. This fundamental characteristic of socks, their inherent "paired-ness," is what gives this joke its delightful foundation (Review). It's a fun bit of anthropomorphism, giving our everyday foot coverings a social life and a desire to celebrate, just like us, but with a unique, sock-centric twist.