From Nookipedia, the Animal Crossing wiki
In Doubutsu no Mori and Doubutsu no Mori e+, the Mushrooming Season occurs during October and lasts for several days. During the Mushrooming Season, four mushrooms can be found around town between 8:00 A.M. and 9:00 A.M.. These mushrooms can be sold to Tom Nook for a hefty price. Every fifteen minutes after 8:00 A.M., one mushroom disappears.
In Animal Crossing: City Folk, the Mushrooming Season occurs from November 1 - November 30. Six mushrooms are usually found at the base of trees each day. One of them will be a furniture piece which is part of the Mushroom Series. The different kinds of mushrooms can be sold to Tom Nook, which players can receive bells depending on the rarity of the mushroom. Rare mushrooms can only be found in a perfect town and are buried in the ground next to trees. They do not disappear when December begins.
The Mushrooming Season returns in Animal Crossing: New Leaf. It is nearly identical to the one in City Folk. However, only five mushrooms appear instead of six.