Some gas stations here in Canada implement their Winter gas blends from November until early March I believe. I know my mileage takes a nose-dive when using those blends and when temperatures get really low (-20F).
35 MPG (winter blend) vs 40 MPG. (normal blend)
Here's the one from Petro-Canada :
http://www.petro-canada.ca/en/wholesalefuel/318.aspx