You didn't undo all your hard work by drinking on Saturday. Unless you drank 3,500 calories worth of alcohol and had not created a deficit earlier in the week.
We tend to think that little things - days - meals - etc - make or break our diets/weight loss efforts. This is simply not true. It's what you do over the long term that counts. That one day of drinking may have been a poor choice but it certainly didn't undo everything.
The most likely scenario is that the alcohol caused you to hold water weight for your weigh-in. Your WI was barely 48 hours afterwards. Hang in there, keep doing good things this week, and I bet the scale will show you some love next WI.