The #CityOfHeroes Homecoming servers have enabled Double XP until at least Tuesday. This is additive with the START vendor's Double XP buff, so if you have that you'll be earning 3x XP.
Also keep in mind that the mobs that come out of doors while Trick or Treating are worth 4x the normal mob XP (versus those in a fire farm, for example). So now is a great time to get some levels on your toons!
(Zombie invasion mobs are worth 1/4 XP, so are not worth farming unless you want badges.)