Yom Yerushalayim (Jerusalem Day) is an Israeli national holiday commemorating the reunification of Jerusalem and the establishment of Israeli control over the Old City in the aftermath of the June 1967 Six-Day War. The day is officially marked by state ceremonies and memorial services. While the day is not widely celebrated outside Israel, and has lost its significance for most secular Israelis, the day is still very much celebrated by Israel’s Religious Zionist community with parades and additional prayers in the synagogue. The Chief Rabbinate of Israel declared Jerusalem Day a minor religious holiday to mark the regaining of access to the Western Wall.
On May 12, 1968, the government proclaimed a new holiday – Jerusalem Day – to be celebrated on the 28th of Iyar, the Hebrew date on which the divided city of Jerusalem became one. On March 23, 1998, the Knesset passed the Jerusalem Day Law, making the day a national holiday.
Source: wikipedia.org
In 2017 Jerusalem Day in USA falls on May 24.