National Beer Day is celebrated in the United States every year on April 7, marking the day that the Cullen–Harrison Act was enacted after having been signed into law by President Franklin D. Roosevelt on March 22, 1933. Upon signing the legislation, Roosevelt made his famous remark, "I think this would be a good time for a beer." The law went into effect on April 7 of that year, allowing people to buy, sell and drink beer containing up to 3.2% alcohol by weight (or 4.05% by volume) in states that had enacted their own law allowing such sales. People across the country responded by gathering outside breweries, some beginning the night before. On that first day, 1.5 million barrels of beer were consumed, inspiring the future holiday. Today, April 7 is recognized as National Beer Day and April 6 is known as New Beer's Eve.
It is important to note that the Cullen-Harrison Act was not the official end of prohibition in the US (that happened on December 5, 1933 when the 21st Amendment was ratified). What the Cullen-Harrison Act did do was redefine an "intoxicating beverage" under the Volstead Act. As such, April 7th is a beer specific holiday and should not be confused with Repeal Day celebrated on December 5th.
National Beer Day was first created in 2009 by Justin Smith of Richmond, VA. After much prodding from his friend, Mike Connolly, Justin started a Facebook page that was noticed by Colorado Beer Examiner, Eli Shayotovich. Justin's promoting of the new holiday via various social media outlets was rewarded when the beer drinking app, Untappd, created a badge for National Beer Day that rewarded participants that checked a beer into the app on April 7th. National Beer Day has since been trending every year on April 7th using the hashtag #NationalBeerDay.