I usually have a drink once a week. I had a peach margarita around 6pm Friday, and had a EtG 80 test today at noon, cut off 100. Will I pass?

Probably. You'll have 66 hours of time which is usually enough one one drink.

The more important question is why you're being randomly tested? What happens if you fail? IF you are being tested, then there has to be reason why you're not allowed to drink. Considering this, why are you drinking? Even if it is only once a week.