Originally coined on the Chicago Board Options Exchange, a Synthetic Friday occurs on a Thursday preceding a Friday in which the market is closed. Since there is no work on Friday, Thursday essentially becomes like a Friday. The word synthetic is used and derived from an option strategy called a Synthetic Long Stock, in which a trader buys a call and shorts a put, each with the same strike price (there are different variations of this strategy/trade.)
Market Maker A: "The market is closed tomorrow since it is Good Friday."
Market Maker B: "Nice, today's a Synthetic Friday."
Market Maker A: "Let's get bombed."