You're definitely being superstitious. Friday the 13th has no actual connection to product quality or how well electronics will work. It's just a date, and bad luck is just a pattern our brains like to see when random things happen. If your monitor breaks down in two months, you won't blame it on a random Tuesday - you'll just assume it was a defect. But buy it on the 13th and suddenly people remember the date. That's confirmation bias, not reality.
The real question is whether the deal itself is good. Check the return policy, the seller's reputation, warranty coverage, all that practical stuff. Those actually matter for whether you'll be happy with the purchase. If it's a legit deal from a solid retailer and the monitor has decent specs for what you need, grab it. Your roommate's just messing with you anyway.