Life with a shedding dog means you've got to watch where you sit for fear of ending up with Fido's fur all over your black pants. While owners like to know what season to expect the bulk of the hair loss, for many the answer is, unfortunately: all of them. Changes to canine lifestyles have decreased seasonal shedding over time.
Many pups have heavy shedding seasons in the spring and fall. The spring shed removes thick winter fur and prepares the pups for summer; fall shedding removes light hair so the thicker winter coat can grow in. These seasonal sheds prepare pups for summer and winter, although they do not occur in those particular seasons. For dogs who shed seasonally, the spring shed is generally heavier as it removes all of that thick winter insulation.
Historically -- and, to a lesser extent, today -- dogs shed seasonally in response to the changing day length. As days grew longer in the spring, dogs would shed their winter fur. As days shortened in the fall, dogs lost their light coats. These circadian rhythms prompted a range of canine changes, including mood and reproductive habits. To an extent, circadian rhythms still control these behaviors. However, since most dogs live domesticated indoor lives, nature's pull is less strong.
Dogs that live indoors receive less natural light than their ancestors did. As a result, they tend to shed throughout the year as opposed to seasonally. Shedding tends to be fairly consistent from month to month, without a particular spike in volume. There's an important distinction between shedding and loss of hair. If you do notice sudden, patchy hair loss in a given season, talk to your vet. Health conditions such as an under-active thyroid gland could cause hair loss.
Certain breeds tend to retain seasonal shedding habits more than others. In particular, so-called Nordic breeds, such as Siberian Huskies and German or Finnish Spitzes, tend to shed biannually, even when they live indoors. Dogs that have undercoats tend to lose -- or "blow out" -- these coats seasonally as well, so you'll notice additional hair around the home at these times.