The Rath Yatra festival is based around the worship of Lord Jagannath (a reincarnation of Lords Vishnu and Krishna). It commemorates his annual visit to his aunt's home.
When is Rath Yatra Celebrated:
On the second day of the Shukla Paksha (waxing phase of moon or bright fortnight) Ashadh month, as per traditional Oriya Calendar. In 2011, Rath Yatra commences on July 3. It runs for 10 days. The main festivities occur on the first day.
Where is Rath Yatra Celebrated:
At the Jagannath temple in Puri, Orissa.
How is Rath Yatra Celebrated:

The exuberant Rath Yatra festival sees Lord Jagannath, along with his elder brother Balabhadra and sister Subhadra, venture out of their abode in the Jagannath temple. The three of them travel to their aunt's Gundicha Temple a short distance away, where they remain for seven days before returning.

The gods are transported on towering chariots, which have been made to resemble temples, giving the festival its name of Rath Yatra -- the Chariot Festival. Around one million pilgrims flock to this colorful event.
What Rituals are Performed During Rath Yatra:

The Rath Yatra is a community festival. People don't worship in their houses or fast.

Every year, three huge new chariots are made for the festival. The deities are bathed, dressed, and placed in their respective chariots. Amidst the beating of drums and gongs, and the blowing of conch shells, thousands of devotees pull the chariots through the streets to the Gundicha Temple. People come from far off places to take part in the event each year.

When the gods return, they're decorated and adorned with ornaments of pure gold, before being placed back inside the Jagannath temple on the tenth day.
An entertaining comic scene is enacted for onlookers, as part of the grand finale.
What Can Be Expected at the Rath Yatra Festival:

The Rath Yatra festival is the only occasion when non-Hindu devotees, who aren't allowed inside the temple, can get their glimpse of the deities. A mere glimpse of Lord Jagannath on the chariot, or even to touch the chariot, is considered to be very auspicious.

The massive number of devotees that flock to the festival does pose a safety risk. Lives are often lost in the immense crowd, so extra care should be taken.
Interesting Information About Lord Jagannath:
The idol of Lord Jagannath doesn't have any arms and legs. Do you know why? It was carved out of wood by a carpenter after the Lord came to the King in a dream, and instructed him to get the idol made. Apparently, if anyone saw the idol before it was finished, the work would not progress any further. The King became impatient and took a peek. Hence, the idol not having arms and legs!