When does it begin and end typically?
The mushroom harvesting season is in its high peak now (Mid September). Depending on the region it starts sometime at the middle of June and lasts till end of October.
However, many people report finding mushrooms in late November and the true experts claim that the mushroom season actually lasts year round, since there are groups of so-called "winter mushrooms", especially those growing on trees.
Example:
2005, December 10, Podkarpackie Voivodship: spotted a late colony of "rydze" (Lactarius deliciosus (L.) Pers.); some of them were half-frozen (minus two centigrades, light snow cover).
One of the latest mushrooms to be harvested belong to the genus Tricholoma (Polish "gąska" - a little goose). Most of the species are edible and some are very delicious, usually used in marinates. But watch it - some of its species are poisonous.