(This is a past event.)
On May 15 each year, more than five hundred people wear ancient costumes and proceed with forty horses and ox and oxcart on the Kyoto city streets. Aoi matsuri festival originated in six century when Emperor send a messenger to the two shrines of Shimogamo and Kamigamo to pray for ending the plague, and today its tradition is still performed. So the festival is relatively quiet and elegant, with no dance or music during the procession, whereas many other festivals in Japan established by the citizen hoping for a good harvest are noisy.
The parade starts at Kyoto Imperial Palace (10:30 a.m.) and goes Marutamachi street to the East, Kawaramachi street to the North and arrives at Shimogamo shrine at 11:40 a.m., then heads to the north along the Kamo river, Kamo-kaido street and ends at the Kamigamo shrine (3:30 p.m.). At Shimogamo shrine, you can watching their beautiful costumes in a fresh greenery backdrop. There is a ceremony offering to deity at the Shrine.
You can view the procession anywhere on the route. But the best place to view the parade is the Imperial Palace and Shimogamo shrine. They have reserved seat (2000 yen) for spectators. It takes about an hour to watch the entire procession passing through. If it rains, rescheduled to the next day.
Festival information (seat reservation only available in Japanese language) http://www.kyokanko.or.jp/3dai/aoi_reserve.html
Shimogamo Shrine http://www.shimogamo-jinja.or.jp/pg150.html
Kamigamo Shrine http://www.kamigamojinja.jp/english/index.html