Various style manuals and guides (MLA, Chicago, Guardian, Grammar Girl) tend to concur:

• The four seasons are lowercased.
• Except when part of a formal name, such as the name of an event (Winter Olympics), school term (Spring 2012), or issue of a journal (Summer 2008).
• Except when the season is personified, as in poetry ("Then Spring--with her warm showers--arrived.").

Various style manuals and guides (MLA, Chicago, Guardian, Grammar Girl) tend to concur:

• The four seasons are lowercased.
• Except when part of a formal name, such as the name of an event (Winter Olympics), school term (Spring Quarter 2012), or issue of a journal (Summer 2008).
• Except when the season is personified, as in poetry ("Then Spring--with her warm showers--arrived.").