Tonight my 7 year old son asked me what season's greetings meant. I wasn't really sure so I consulted ye olde Wiki. There was quite a lengthy spiel about it but one bit  made me laugh:

"Season's Greetings" is grammatically erroneous and ambiguous, as the greeting can equally (apply) to any other season. The definition of "season" is also somewhat subjective, as any time frame may be labelled a season and may have a set of celebrations culture and greetings, etc. Grammatically, a Season is inanimate and thus it cannot greet you, thus it cannot possess, thus the possession marked by the apostrophe is erroneous. Semantically, a more grammatically correct rendition would be:

"Wishing you Greetings of the Season"