[professionall2009]

If we follow the British English, the answer will be" for " but if we follow the American English both "for" and "since" are correct as mrososeng said. That's because in British English, the seasons of the year are not preceded by " the " so " since is not correct here.But in American English they are preceded by "the" so both "for" and "since" are correct. That's my opinion. I hope you discuss it with me