From tonight until tomorrow evening, we celebrate Jerusalem Day or Yom Yerushalayim, which marks the liberation of Jerusalem in 1967. This is a particularly significant event for Jews because:
The destruction of Jerusalem was a watershed event in Jewish history that began thousands of years of mourning for Jerusalem, so, it follows, that the reunification of Jerusalem should be a joyous celebration that begins the process of reversing thousands of years of destruction and exile. Jerusalem is central to the Jewish tradition. Jews face in the direction of Jerusalem and all of the prayer services are filled with references to Jerusalem.
Contrast this to Muslim prayers which hardly ever mention Jerusalem (if at all).
Here is more on what happened on that fateful day in 1967.
Here are some great pictures of Jerusalem.
And here you can take a 3D tour of Jerusalem.