Following the creation of the state of Israel in 1948, it had been agreed that Jerusalem would be an international city for ten years, after which time the residents could vote for what country they wanted to join. However, Israel was attacked by Jordan in May 1948 resulting in parts of Jerusalem being under Israeli control and parts under Jordanian rule. 

On June 6th 1967, the Six-day war began. On June 7th (28th of Iyar), the Jordanian controlled part of Jerusalem was taken by Israeli forces. The war ended with a cease-fire on June 11th 1967. 

In May 1968, The 28th of Iyar was proclaimed as a holiday to mark the unification of Jerusalem. 

In 1998, the Israeli government passed the Jerusalem Day Law, making this day a national holiday. 

Note: Like other Jewish holidays, Yom Yerushalayim will begin at sundown on the previous day.