Typically, Yom Kippur, the Day of Atonement is the holiest (and most somber) day of the year as Jews are asking God to absolve their sins of the previous year.
Judaism.See also:3800-year history of the Jews in IsraelThe Holy City
It depends on your religious belief.
Yom Kippur
In 539 B.C.E the persians conqured Babylon freeing most Jews from exile then the Romans conquered the Jews in later stages.
.Roman Catholic AnswerThe three most Holy Days of the year are usually considered Easter, Pentecost, and Christmas.
All year round, but especially at Easter and Christmas, thousands of people go to the Holy Land on pilgrimages.
In the year 70 C.E. (Common Era), the Romans destroyed Jerusalem and its Holy Temple and exiled the Jews. However, although Jews were no longer permitted in Jerusalem, Jews lived throughout the country. In fact, there was never a period where there were NO Jews living somewhere in Israel. During the Middle Ages, more Jews began to return to Israel, growing in the Nineteenth Century. By the end of the Nineteenth Century, Jews outnumbered Arab in Jerusalem itself.
Holy Week is Holy Week all over the world. It is the holiest and most solemn week of the Liturgical Year when the passion and death of Our Lord are remembered.
Jews were commanded to go there three times a year (Deuteronomy ch.16).
Jewish people go to synagogue, there are no churches in Judaism. Religiously observant Jews pray 3 times a day, every day and 4 times on Saturday. Not all of these prayer sessions require attendance at a synagogue, most just require a quorum of 10 or more men.
Holy Week, ending with Easter, is the holiest time of the liturgical year, with Christ Death and Resurrection being the climax.
Most people consider Christmas to be the best holiday of the entire year.