Season 3, Episode 7: "The Bracebridge Dinner": This episode features Rory's first experience at a formal wedding, where she is the Maid of Honor for her best friend Lane.
Season 4, Episode 22: "Raincoats and Recipes": This episode features Lorelai and Sookie's wedding planning for their friend Jackson and his fiancee, as well as Rory's own first real date with Logan.
Season 5, Episode 7: "You Jump, I Jump, Jack": This episode features the actual wedding of Jackson and his fiancee, and it is shown as a very chaotic event.
Season 7, Episode 1: "The Long Morrow": This episode features Lorelai and Luke's wedding, which is the most significant wedding event in the entire series.
Other episodes may also have wedding-related subplots, but these are the most prominent episodes focused on weddings.