Did you see it? It reads "Celebrating the repeal of the 21st Amendment." It's the 21st Anniversary of the start of the fest, and being a little too clever, organizers must have thought they'd go for a triple-entendre: 21 is the fest's age, also the legal drinking age, and the 21st Amendment created prohibition. So organizers determinded that this year's theme would be the repeal of the 21st. Except that the Amendment that created Prohibition was the 18th.
The 21st Amendment repealed the 18th:
Section 1. The eighteenth article of amendment to the Constitution of the United States is hereby repealed.Apparently, organizers are "aware of the problem" and we should expect to see a revision to the poster.
Section 2. The transportation or importation into any State, Territory, or possession of the United States for delivery or use therein of intoxicating liquors, in violation of the laws thereof, is hereby prohibited.