Crowley and Aziraphale get an invitation to attend Adam's Halloween Party the October after the world didn't end. Crowley is less than enthused, at least about the 'having to dress up' part. Aziraphale seems much more interested, however, and, with a bit of help, finds something for both him and Crowley to wear. Something quite...different, to say the least.
It might even push them a bit further along in their relationship.