Check out this article about mapping building shadows of New York City.  Building shadows were measured using a shadow accumulation technique and the dates of December 21 (winter solstice), June 21 (summer solstice), and September 22 (autumnal equinox) were chosen for the shadow profile.  Shadows can be greatly distorted when they are near the horizon, so all the shadows in the model were estimated from one and a half hours after sunrise and one and a half hours before sunset.

Thanks goes to Mary Bates from College of the Canyons for finding this article!