According to Al Korzonas in his excellent book Homebrewing, Vol. 1:
Bière de Garde means "beer to keep" implying that it is meant to be stored. This dates back to the days when summer brewing was impractical and a stronger beer was brewed in the spring to be stored for summer consumption.
They are typically malty, expressive, fruity, and rich. Sounds perfect for an evening in mid-May, when the season has warmed but there’s still a frosty chill to the air.
Aside from Biere de Mars, some other Bière de Gardes to look for are: Jenlain, from Brasserie Duyck; Perdition from Russian River Brewing; and Oro de Calabaza from Jolly Pumpkin Artisan Ales. (Okay, I cheated a bit and pulled those from BeerAdvocate’s list.)