Hollywood loves Halloween!
In one shot, Zac—who completed a rehab stint this past April—hangs onto a candy apple and his pal Brittany. (Don't start playing the couples game here! Britt is dating Teen Wolf's Tyler Hoechlin.)
Derek, whose girlfriend Nina Dobrev did not appear to be part of the festivities, Instagrammed a smiling, happy shot of himself and his famous friends. "Great time last night with great friends," he wrote Friday. "Love this time of year."