Businesses and retailers have been using seasonal sales tactics for years. One of the most recent and cult-like is Starbucks Pumpkin Spice Latte first introduced in 2003. Before the craze became a pop culture phenomenon, pumpkin spice was primarily a thing reserved for pies. But demand kept on dictating supply. Today, you can purchase pumpkin spice condoms.
Pumpkin spice has become such a phenomenon, it carries with it SEO goodwill. By this time, Pumpkin spice has joined eggnog and turkey. You can't buy that kind of publicity.
Who Are You?
It's doubtful that your firm will ever be as recognized as the seasonal Starbucks latte. But an increasing number of firms have started offering services that are limited to a particular time of year.
Seasonal offerings of particular services only work if people are aware of the kinds of services you are providing. It's fair to say that Yelp and other review services have supplanted much word of mouth. Your firm should already be firmly established on the social media waves and should have a strong local business presence. If you don't know that you're at the mercy of Google+ and Pigeon, you're already in trouble.
Pro Bono Presence
Small firms stand to benefit from offering some of their services gratis -- both as a community benefit and also as a way to establish a community presence. Some of the best times to advertise in the local area is during the less ballyhooed "recognition periods" such as Veteran's day, Breast Cancer Awareness Month, Domestic Violence Awareness Month, the National Depression Screening day, etc. Obviously, these don't get the billing of Christmas or tax season, but they still present opportunities.
Tailor your services so that you offer services that are themed for those particular days. A domestic violence client might be offered services at a discount in recognition of the month.
A driving factor in why seasonal goods and services work for marketing has to do with their limited nature. Of course, a lawyer's services in a domestic violence case should not be treated like a commodity like pumpkin spice, but the holiday will provide your firm with a welcome excuse to market services without appearing to be an overt business attempt.
Doing this consistently may provide an opportunity to create a tradition. For example, doctors send annual reminders of checkups to their patients. The point, however, is to keep services limited such as to drum up client anticipation for your firm's services.