The answer really is 'no' to this one. It will likely slow down your weightloss, yes. But if it's a special occasion then i'd say go for it, just try to moderate how much you have. The main problem with alcohol, besides any carbs in the drink, is that it is heavy on your liver and instead of metabolising fat your liver has to divert all it's resources to dealing with removing alcohol from your bloodstream instead. Besides the fact that this means fatburning could be put on the backburner by your body, it's just pretty hard going on your body/liver/kidneys - especially if you are in the first couple of weeks. Having said all that if it's a one off then i wouldn't worry, just prepare to be set back by a couple of days.