Jenna's Barbecue was an event that was held in 2010.
Jenna Sommers organized it and held it at her house. Technically, it was Damon's idea to take advantage of the fact that Jenna and Mason went to high school together. With a purpose of knowing more about Mason, he told Alaric about the idea and Alaric told Jenna to make it happen.
Jenna introduced Mason to Damon and Alaric as an old friend from high school. Jenna told Mason that the barbecue was Alaric's idea. Alaric told Mason that it's for digging some dirt of Jenna's past. They later played Pictionary, Damon was drawing a dog with a skirt, Jenna said it was a "dress, ballerina", Caroline said "puppy with a chuchu", Mason said it was a "wolf", then finally Damon said he was correct.
Elena and Caroline left the barbecue early because Elena was worried about Stefan since she wasn't able to reach him for the whole time. Caroline offered to drive her to him but had the real intention to occupy her because Katherine ordered her to.
Damon made several indications to Mason that he knows about him being a werewolf. Later, they talked in private and Mason offered Damon peace. Damon accepted the offer, but later followed him to kill him, which he did not manage to do.