"Happy Days Are Here Again" is a song copyrighted in 1929 by Milton Ager (music) and Jack Yellen (lyrics) and published by EMI Robbins Catalog, Inc./Advanced Music Corp. The song was recorded by Leo Reisman and His Orchestra, with Lou Levin, vocal (November 1929), and was featured in the 1930 film Chasing Rainbows. The song concluded the picture, in what film historian Edwin Bradley described as a "pull-out-all-the-stops Technicolor finale, against a Great War Armistice show-within-a-show backdrop." This early example of 2-strip Technicolor footage was, along with another Technicolor sequence, later cut from the 1931 re-edited release of the otherwise black-and-white film, and is believed to have been lost in the 1967 MGM Vault 7 fire.

Today, the song is probably best remembered as the campaign song for Franklin Delano Roosevelt's successful 1932 presidential campaign. According to Time magazine, it gained prominence after a spontaneous decision by Roosevelt's advisers to play it at the 1932 Democratic National Convention, and went on to become the Democratic Party's "unofficial theme song for years to come". The song is also associated with the Repeal of Prohibition, which occurred shortly after Roosevelt's election where there were signs saying "Happy days are beer again" and so on.

In the British radio comedy series "I'm sorry I'll read that again" HDAHA is often sung on the entrance of the appalling Lady Constance.

Matthew Greenwald described the song as "[a] true saloon standard, [and] a Tin Pan Alley standard, and had been sung by virtually every interpreter since the 1940s. In a way, it's the pop version of Auld Lang Syne."

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