Stuff that captures the essence of a particular season (could be just for you personally or on a more universal level). Not limited to one per season.
e.g. for me so far...
Spring: Fleet Foxes - Fleet Foxes
Summer: ???
Fall: ???
Winter: Björk - Vespertine
There's been some stuff like AnCo where I thought "Hey this could be my summer album" but it hasn't really clicked with me yet like Vespertine has.
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Furious Fura has the best music taste.