The Constitution of May 3, 1791 is generally regarded as Europe's first and the world's second modern codified national constitution, following the 1788 ratification of the United States Constitution. The May 3, 1791, Constitution was adopted as a "Government Act" on that date by the Sejm (parliament) of the Polish–Lithuanian Commonwealth. It was in effect for only a year, until the Russo-Polish War of 1792.

The May 3 Constitution was designed to redress long-standing political defects of the Polish–Lithuanian Commonwealth and its traditional system of "Golden Liberty" conveying disproportionate rights and privileges to the nobility. The Constitution introduced political equality between townspeople and nobility (szlachta) and placed the peasants under the protection of the government, thus mitigating the worst abuses of serfdom. The Constitution abolished pernicious parliamentary institutions such as the liberum veto, which at one time had put the sejm at the mercy of any deputy who might choose, or be bribed by an interest or foreign power, to undo legislation passed by that sejm. The Constitution sought to supplant the existing anarchy fostered by some of the country's magnates with a more democratic constitutional monarchy.

The adoption of the May 3 Constitution provoked the active hostility of the Commonwealth's neighbors. In the War in Defense of the Constitution, the Commonwealth was betrayed by its Prussian ally, Frederick William II, and defeated by Catherine the Great's Imperial Russia allied with the Targowica Confederation, a cabal of Polish magnates and landless nobility who opposed reforms that might weaken their influence. Despite the Commonwealth's defeat and the consequent Second Partition of the Polish–Lithuanian Commonwealth, the May 3 Constitution influenced later democratic movements. It remained, after the demise of the Polish Republic in 1795, over the next 123 years of Polish partitions, a beacon in the struggle to restore Polish sovereignty. In the words of two of its co-authors, Ignacy Potocki and Hugo Kołłątaj, it was "the last will and testament of the expiring Motherland."

Constitution Day was first declared a holiday on May 5, 1791. Banned during the partitions of Poland (though still then occasionally celebrated), it was celebrated in the Duchy of Warsaw. It was again made an official Polish holiday in April 1919 under the Second Polish Republic—the first holiday officially introduced in the Second Polish Republic. The May 3 holiday was banned once more during World War II by the Nazi and Soviet occupiers.

After the 1946 anti-communist student demonstrations, May 3 Constitution Day lost support with the authorities of the Polish People's Republic, who replaced it with May 1 Labor Day celebrations; in 1951, May 3 was officially rebranded Democratic Party Day and officially removed from the list of national holidays. Until 1989, May 3 was a common day for anti-government and anti-communist protests.

Polish Constitution Day has been a focal point of ethnic celebrations of Polish-American pride in the Chicago area, going back to 1892. Poles in Chicago have continued this tradition to the present day, marking it with festivities and the annual Polish Constitution Day Parade; prominent guests nationwide, most notably Bobby Kennedy, have attended over the years as a way to lobby Chicago Polonia. The anniversary of the May 3, 1791 Constitution has also for decades been observed in San Francisco with celebrations in Golden Gate Park.

May 3 was restored as an official Polish holiday in April 1990, after the fall of communism. In 2007, May 3 was in addition declared a Lithuanian national holiday. The first joint celebration by the Polish Sejm and the Lithuanian Seimas took place on May 3, 2007.