There were three festivals in ancient Israel that required (or at least strongly recommended) a trip to Jerusalem. These were the Passover, Feast of Weeks, and the Feast of Booths.

Passover was, of course, was to commemorate the manner in which the Lord spared the Israelites when he was striking the Egyptians (see Exodus).
Passover took place on the tenth day of the first month of the Hebrew calendar (which is equivalent to our late March to early April). During this festival, a sacrificial lamb was slaughtered (all of the meat must be eaten on that one day according to custom).

The second was the Feast of Weeks (or Pentecost [Shavout]). This occurred seven weeks after Passover, and was a day of sacred assembly that required that no work be done. The primary focus of this festival was thankfulness to God (usually for the wheat harvest). There were detailed list of what be offered to God on behalf of the nation.

The final one was The Feast of Booths (also called Feast of Tabernacles or Sukkoth). This final feast was important as it was very close to two other important days in the Jewish year.
The Feast of Booths took place on the fifteen day of the seventh month, which was called Tishri (equivalent to our late Sept. to early Oct.).
Tishri had the Feast of Trumpets (first day of month), Day of Atonement (tenth day of month), and the Feast of Tabernacles (fifteenth day of month).
For the Feast of Booths, people camped out in small huts for five days (day of atonement to first day of feast of booths) to recall the 40 years spent in the desert. This week was a time of final celebration and thanksgiving for the year's harvest.