at,on介词疑问my friends and i always dress up at halloween.这里为什么

My friends often dress up ___costumes at Halloween. People often dress up at Halloween. Halloween people dress and to in at up go costumes usually p I want him dress up as a ghost at Halloween ? 对吗 一条 Kitty,don't forget to dress up at Halloween.( ),I won't.填 At halloween ,he often likes ( )(dress) up as a monkey the American chlidren always ________ （装扮）up at Halloween. we like ______ ghosts at Halloween A dress up B dressing up My friends and I were all at the fair中译英 So i usually visit my relatives and friends at( ).填空 Dress up at Halloween wear special costumes with masks or _o 请教“What do you do at Christmas"这里的介词为什么用at 而不是用on?