Welcome to the forum.
Treat days are not a bad thing and I certainly never missed out on celebrations/parties
the day after weigh-in is a good time IMO, I changed my weigh-in days to Fridays for this reason
before I would have a treat day (including alcohol) and lost anything from 1-4.5lbs a week, less the more I lost in time.
I always put myself in extra good mode the rest of the week. Just be sensible with syns but don't deprive yourself... my 'treats' in the week to keep me from naughty things were things like:
Sugar free jelly and squirty low fat cream
Muller and strawberries/raspberries
^ both of the above I would sometimes add a small amount of chocolates too (mini pack size amount)
Options hot chocolate, sometimes with some mini marshmallows
I am sure you will be fine, good luck and let us know how you get on