YOM YERUSHALAYIM (Jerusalem Day):

Yom Yerushalayim - Jerusalem Day--is the most recent addition to the Hebrew calendar. It is celebrated 28 Iyar (six weeks after the Pesach Seder, one week before the eve of Shavuot). Although Jerusalem has been considered the capital city of the Jewish people since the time of King David, who conquered it and built it as the seat of his monarchy, there has never been a special day in honor of the city until the Israeli army took over the ancient, eastern part of the city on the third day of the Six-Day War in June 1967.
 Due to the young age of this holiday, there is still not much which makes it unique in terms of customs and traditions. It is gradually becoming a "pilgrimage" day, when thousands of Israelis travel (some hike!) to Jerusalem to demonstrate solidarity with the city.
This show of solidarity is of special importance to Israel, since the international community has never approved the "reunification" of the city under Israeli sovereignty, and many countries have not recognized Jerusalem as the capital of the Jewish State (The United Nations "partition plan" of November 1947 assigned a status of "International City" to Jerusalem).
The Israeli education system devotes the week preceding this day to enhancing the knowledge of the history and geography of the city, with a special emphasis on the unique role that it played in Jewish messianic aspirations since Biblical times.
The ambiguity of the religious status of this holiday is reflected in celebrations, or lack thereof, outside of Israel. While the city of Jerusalem has significant meaning for all Jews, Yom Yerushalayim has yet to obtain the popularity of Yom Ha'atzmaut and is not observed extensively outside of Israel.
In addition, unlike Yom Ha'atzmaut, which is a day to celebrate the existence and successes of the modern Jewish state,Yom Yerushalayim can make some politically liberal Jews outside of Israel uncomfortable, due to the continuing conflicts over the future of the city.

Even some Jews who believe that the city should remain undivided and under Israel's control choose not to emphasize Yom Yerushalayim as a day of joy because of the deeply emotional, violent, and controversial state of affairs surrounding the Arab portions of Jerusalem. Others, however, believe that despite the current political conflicts, an undivided Jerusalem is something to be celebrated openly and unhesitatingly, a sign like Yom Ha'atzmaut of Jewish political independence.
A common citation in Yom Yerushalayim celebrations in Israel is the quote (Psalm 122:4) Ir shehubrah lah yahdaiv-- "a city that is compact together" or "a city uniting all." (This translation is probably influenced by a rabbinic Midrash on this verse which interpreted the phrase to reflect events in rabbinic times. In using the citation today, a modern Midrash has been built on the rabbinic interpretation.)
The course which Yom Yerushalayim will take in future decades will be influenced, undoubtedly by the political developments which will determine the status of the city in future times.

 References:

Falcon PhD, Rabbi T and Blatner, D. (2001) Judaism for Dummies, Hungry Minds, New York.

www.chabad.org/holidays