A special occasion in which, as a bro, you are required to attend by your fellow bro's. this may include a bro's birthday, bro's bachelor party, bro's wedding, but does NOT include any occasion that would require you to see them naked. (Bro's colonoscopy, Bro's nude photoshoot etc.)
Stinson: hey, of course i will be at your bachelor party, it's a special broccasion
Mosby: uh thanks barney, but i don't trust you when it comes to bachelor parties.
Stinson: you owe me after i went to that other "special Broccasion".
Mosby: i told you not to attend my photoshoot
Stinson: ....... the photographer chick was hot