You need to do all the previous quests (check the main quest in journal), and all Fiona's quests, then you should speak to Fiona about halloween party, then buy one (!) costume from her (in the kiosk). Then you should put on this costume (in the dorm's wardrobe) on Friday night or just choose 'skip to Friday night'.
Based on which costume you chose, the halloween story will be different. You'll get close with one girl and the other two will be locked/stuck forever (in this update). If you buy all three costumes, the game will choose superhero costume automatically, so you'd better pick only one costume and save the game before you buy it.