I've gotten several cards with ability triggers involving "summer" and "winter". Though, I have no cards that change the seasons. What am I missing?

You're missing chapters 1 and 2 from the A Time of Ravens cycle. In those chapters you will find Black and White Ravens, which change the season to Summer or to Winter.

Good to know! Is there a source to find what cards are found in the different chapter packs and expansions?

This card search site is an invaluable resource. The tzumainn site is great too, but sadly isn't quite up to date at the moment.