For some stuff that hasn't been listed that I feel captures the essence pretty well...
Fall:
Death Cab for Cutie - We Have The Facts And We're Voting Yes
Frightened Rabbit - The Midnight Organ Fight
Modest Mouse - ...pretty much their entire discography
Winter:
The Weakerthans - Reunion Tour
Spring:
The Applies In Stereo - New Magnetic Wonder
The New Pornographers - Pretty much their entire discography too
Tokyo Police Club - Elephant Shell
Los Campesinos! - Hold On Now, Youngster...
Summer:
Surfer Blood - Astrocoast
Ra Ra Riot - The Orchard
The Pixies - Doolittle
Los Campesinos! - We Are Beautiful, We Are Doomed
Probably worth noting that I'm pretty heavily influenced by when I first listen to an album. I made the mistake of listening to Best Coast and Wavves nearly exclusively for a December and now, as much as I know they sound at least as summer-y as Surfer Blood, they just can't escape the mental association.
On a separate note, I feel that there are a lot of albums that straddle the transition between Summer and Fall; the end of the dog days (at least in the NW United States) is a very distinct period from all other seasons and is I would identify as uniquely notable among seasonal transitions.
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