Just for consistency sake. If you want to have a true picture of your audience, cookies work best when they're as close to "eternal" as possible. Especially for a seasonal" site. Imagine that the site is devoted to superbowl alone - what good will a cookie be there, if its lifetime is less than one year?
I'm most worried that some browsers might not accept cookies with the expiration date set for too long. I know it's not a standard and should not happen, but I was asking if anybody had any headaches with cookies in practice.