Matsuri (祭り, or sometimes just 祭) are a kind of festival or holiday that take place in Japan. They are characterized by being local and usually sponsored by a local shrine or temple.
There is not a specific 'matsuri' day for all of Japan, but vary from area to area. Almost everywhere has at least one matsuri in late summer/early autumn, usually related to the rice harvest.
Some examples are the Jidai, Aoi and Gion Matsuri held in Kyoto, the Ise-machi Matsuri of Nakanojo, Gunma, and the Kanda , Sanno and Sanja Matsuri of Tokyo.
