Because it's a fitting way to contrast the shortest day of the year and herald a new season:

One of the photos TJ sent me from this past spring's trip to Italy, a frieze of bunnies eating grapes at an ancient Roman theatre museum in Verona. He said that above the rabbits were dolphins and that the accompanying placard detailed how Romans believed the latter represented the soul's trip to the sea. TJ said there was no description of what the bunnies symbolized and deduced, "...general coolness, I presume."