Nakba Day (Arabic: يوم النكبة Yawm an-Nakba, meaning "Day of the Catastrophe") is generally commemorated on 15 May, the day after the Gregorian calendar date for Israeli Independence Day (Yom Ha'atzmaut). For the Palestinians it is an annual day of commemoration of the displacement that followed the Israeli Declaration of Independence in 1948.

During the 1948 Palestine War, an estimated 700,000 Palestinians were expelled or fled, and hundreds of Palestinian villages were depopulated and destroyed. The vast majority of Palestinian refugees, both those outside the 1949 armistice lines at the conclusion of the war, and those internally displaced, were barred by the newly declared state of Israel.^{[citation needed]}

These refugees and their descendants number several million people today, divided between Jordan (2 million), Lebanon (427,057), Syria (477,700), the West Bank (788,108) and the Gaza Strip (1.1 million), with at least another quarter of a million internally displaced Palestinians in Israel. The displacement, dispossession and dispersal of the Palestinian people is known to them as an-Nakba, meaning "catastrophe" or "disaster”.

The 1948 Palestinian exodus, known in Arabic as the Nakba (Arabic: النكبة‎, an-Nakbah, lit. "disaster", "catastrophe", or "cataclysm"), occurred when approximately 711,000 to 725,000 Palestinian Arabs left, fled or were expelled from their homes, during the 1948 Arab-Israeli War and the Civil War that preceded it. The exact number of refugees is a matter of dispute. The causes remain the subject of fundamental disagreement between Arabs and Israelis.

Nur-eldeen Masalha writes that over 80 percent of the Arab inhabitants left their towns and villages in 1948, while Rashid Khalidi puts the percentage at 50. Factors involved in the flight include Jewish military advances, attacks against Arab villages and fears of massacre after Deir Yassin, which caused many to leave out of panic; expulsion orders by Zionist authorities; the voluntary self-removal of the wealthier classes, the collapse in Palestinian leadership, and an unwillingness to live under Jewish control,. Later, a series of laws passed by the first Israeli government prevented them from returning to their homes, or claiming their property. They and many of their descendants remain refugees. Later in the war, Palestinians were expelled as part of Plan Dalet. The expulsion of the Palestinians has since been described by some historians as ethnic cleansing, while others dispute this charge.

A day is a unit of time, commonly defined as an interval equal to 24 hours. It also can mean that portion of the full day during which a location is illuminated by the light of the sun, also known as daytime. The period of time measured from local noon to the following local noon is called a solar day.

Several definitions of this universal human concept are used according to context, need and convenience. In 1967, the second was redefined in terms of the wavelength of light, and it became the SI base unit of time. The unit of measurement for time called "day", redefined in 1967 as 86,400 SI seconds and symbolized d, is not an SI unit, but it is accepted for use with SI. A civil day is usually also 86,400 seconds, plus or minus a possible leap second in Coordinated Universal Time UTC, and, in some locations, occasionally plus or minus an hour when changing from or to daylight saving time. The word day may also refer to a day of the week or to a calendar date, as in answer to the question "On which day?" Day also refers to the part of the day that is not night — also known as 'daytime'. The life patterns of humans and many other species are related to Earth's solar day and the cycle of day and night (see circadian rhythms).