Season 3, Episode 13 — “Shut the Door. Have a Seat.”
In one of the greatest Mad Men episodes ever, Sterling Cooper Draper Pryce is formed via clandestine meeting late at night. Joan is asked to join the new agency, and shows up to the office for said meeting looking chic as ever. This is the only time Joan wears pants to the office, but with how Audrey Hepburn classic she looks, it should really be considered more often, despite how un-Joan-like it may be.