I am continually asked this question, so here is a very detailed answer. The answer here applies to southern England, and should be adjusted for latitude — in northern Scotland autumn and winter come at least a month earlier, and further south in continental Europe they come later (unless you are in the mountains….).
Fungi, including edible species, can be found at any time of year. They are scarcest in the depths of winter and during drought conditions that most frequently occur during the summer. There is a brief flurry of activity in the spring, usually peaking in April or May, when spring-fruiting specialists such as St George’s Mushrooms and Morels can be found. Other species, such as Chicken of the Woods, Dryad’s Saddle and Field Mushrooms can fruit at any time from spring until early autumn.
The vast majority of fungi fruit between late summer and early winter, but each year is different and their behaviour is extremely hard to predict. In an ideal autumn, we would get a decent amount of rain once or twice a week, as the average temperature steadily drops. If this were to happen, then the main mushroom season would start some time in August, build to a peak around the end of September and start of October, and fizzle out with the first ground frosts in December. In reality, of course, this never happens. Instead we get the British weather, which can include summers when the sun never shines, followed by high temperatures and drought conditions in October. Other factors affecting the availability of edible fungi include what happened last year (if a particular species had a great year last year, maybe it will take a year off), how good a year the trees have had (this affects the symbiotic mycorrhizal fungi), and the size of the slug population.
You can be forgiven for thinking that the season typically starts slowly, ramps up to a peak, then ramps down again. This is rarely what happens. Very often the season will start with a big burst of activity from symbiotic mycorrhizal fungi. These include many of the very best species, such as Penny Buns and Chanterelles, and it frequently occurs right at the start of the season, sometimes as early as the second week in August. In other years, especially during dry summers, there is very little to be found until the middle of September. Whenever there has been a dry period, there will be a burst of activity after the first proper deluge, but it takes between ten and twenty days for the fungi to respond. In exceptionally dry autumns, this peak of activity can be delayed until early November.
Later in the autumn the saprophytes/parasites start to take over from the mycorrhizal fungi, and this seems to be particularly true in coniferous woodland and unimproved grassland. Edible waxcaps, as well as (mycorrhizal) Winter Chanterelles can be found in late November, or even well into December. The main fungi fruiting season is brought to a close by the first serious ground frost, after which the only things that continue to appear are cold weather specialists like Velvet Shanks and some hardy all-year-rounders like Judas’ Ear (Jews Ear).
Fungi foraging is, therefore, a lottery. Not only are the fungi unpredictable, but I frequently find myself struggling to explain why they’ve done what they’ve done even after they’ve done it. This unpredictability and mystery is part of their charm.