Basically a way of saying to someone (truthfully) you think they are very attractive. Usually used on myspace comment pictures to say that the person looks good, but you are not just saying it to get a comment back with the same thing about you.
Lemon season is the state of mind more commonly called, "being stoned". If it's lemon season you are smoking weed pretty much all night or day. This expression was made famous in the Roll Deep track, "weed man".
Guy 1: You coming round mine tonight? It's lemon season bro!