The most miserable month ever according to the Misery Index was June of 1980. The all-time high for the Misery Index was reached on June 1, 1980, when it hit 21.98%.

The Misery Index was devised by the economist Arthur Okun in the 1960s. Okun, an advisor to President Johnson, wanted a simple way to express the social costs of policy in terms of inflation and unemployment. To achieve this, he added the rates of inflation and unemployment together and called the sum the Misery Index.

From the 1960s to the '80s, oil cartels, the Cold War and the Vietnam War all took a toll on the economy, driving up inflation and unemployment. When Gerald Ford ran for re-election, his Democratic opponent, Jimmy Carter, used the high Misery Index of 13% as proof of Ford's inability to steer the economy. Carter was elected president, but he unwittingly tied the rope from which he'd later hang.

Under the Carter administration, a taxing regime and price controls were implemented. While this slowed inflation, it also meant that businesses were given less incentive to profit and, thus, less incentive to hire. Unemployment rose sharply and the Misery Index hit an all-time high of 21.98% during Carter's presidency.

After having called attention to the index through his own election campaign, Carter tried to minimize the index's importance during his re-election campaign. This strategy failed and Reagan was elected president, ushering in his own solutions in the form of Reaganomics.

For related articles, read Economic Indicators: Overview and Laffer Curve Key to Ideal Tax Rate.

This question was answered by Andrew Beattie.