Nakba Day (Arabic: ذكرى النكبة, romanized: Ḏikrā an-Nakba, lit. 'Memory of the Catastrophe') is the day of commemoration for the Nakba, also known as the Palestinian Catastrophe, when the Palestinian homeland and society were destroyed in 1948 and most Palestinians were permanently displaced. It is generally commemorated on 15 May, the day after the Gregorian calendar date of the Israeli Declaration of Independence on 14 May 1948.

The day was officially inaugurated by Yasser Arafat in 1998, though the date had been unofficially used for protests since as early as 1949.

Nakba Day is generally commemorated on 15 May, the day after the Gregorian calendar date for Israel's Independence. In Israel, some Arab citizens have held Nakba Day events on Yom Ha'atzmaut (Israel's Independence Day), which is celebrated in Israel on the Hebrew calendar date (5 Iyar or shortly before or after). Because of the differences between the Hebrew and the Gregorian calendars, Independence Day and 15 May only coincide every 19 years.

As early as 1949, the year after the establishment of the State of Israel, 15 May was marked in several West Bank cities (under Jordanian rule) by demonstrations, strikes, the raising of black flags, and visits to the graves of people killed during the 1948 war. These events were organized by worker and student associations, cultural and sports clubs, scouts clubs, committees of refugees, and the Muslim Brotherhood. Speakers at these gatherings blamed Arab governments and the Arab League for failing to "save Palestine", according to author Tamir Sorek. By the late 1950s, 15 May was known in the Arab world as Palestine Day, mentioned by the media in Arab and Muslim countries as a day of international solidarity with Palestine.

Commemoration of the Nakba by Arab citizens of Israel who are internally displaced persons as a result of the 1948 war has been practiced for decades, but until the early 1990s was relatively weak. Initially, the memory of the catastrophe of 1948 was personal and communal in character and families or members of a given village used the day to gather at the site of their former villages. Small-scale commemorations of the tenth anniversary in the form of silent vigils were held by Arab students at a few schools in Israel in 1958, despite attempts by the Israeli authorities to thwart them. Visits to the sites of former villages became increasingly visible after the events of Land Day in 1976. In the wake of the failure of the 1991 Madrid Conference to broach the subject of refugees, the Association for the Defense of the Rights of the Internally Displaced was founded to organize a March of Return to the site of a different village every year on 15 May to place the issue on the Israeli public agenda.

By the early 1990s, annual commemorations of the day by Palestinian Arab citizens of Israel held a prominent place in the community's public discourse.

Meron Benvenisti writes that it was "Israeli Arabs who taught the residents of the territories to commemorate Nakba Day." Palestinians in the occupied territories were called upon to commemorate 15 May as a day of national mourning by the Palestine Liberation Organization's United National Command of the Uprising during the First Intifada in 1988. The day was inaugurated by Yasser Arafat in 1998.

The event is often marked by speeches and rallies by Palestinians in Israel, the West Bank and Gaza, in Palestinian refugee camps in Arab states, and in other places around the world. Sometimes clashes develop between Palestinians and the Israel Defense Forces in the West Bank and Gaza Strip. In 2003 and 2004, there were demonstrations in London and New York City. In 2002, Zochrot was established to organize events raising the awareness of the Nakba in Hebrew so as to bring Palestinians and Israelis closer to reconciliation. The name is the Hebrew feminine plural form of "remember".