Interesting idea. Do you use the Ritual spells from Relics and Rituals? The Druid ones were also season oriented.
Somehow, gaining a bonus 1/4 of the year, and a penalty 3/4 of the year doesn't seem quite right.
How about having it center around the solstices and the equinoxes. The druid picks one (Winter or Summer Solstice, Spring or autumn equinox) and on that particular day he gets +2 to the DC. Half of the year he gets +1 (between the equinoxes or the solstices depending on his choice). On the type of day not picked (Solstices if an equinox is picked, equinox if a solstice is picked) the DC is +0, the other half of the year the DC is at -1, and on the opposite day (autumn and spring equinoxes opposite, winter and summer solstices opposite.)
An example just in case you didn't follow that.
Bob the Druid takes this feat. Bob believes the Winter Solstice, the day of his birth, is the most holy of days and chooses that day to be at his best. He has +2 to all DCs on that day. From the Autumn Equinox to the Spring Equinox, Bob has +2 to all his DCs. On the equinoxes themselves, Bob gets +0. Between the Spring Equinox and the Fall Equinox Bob has -1 to all his DCs. On the dreaded Summer Solstice Bob has -2 to all his DCs.
Ya, it's a little complex and the DM has to keep track of days. But I think it fits a little better.