Ahoy!
There are a lot of card abilities say " if its winter" or "if its summer", or contains both. For example Ice Fisherman`s ability: "If it is Winter,Ice Fisherman gains a power icon and: 'Marshalling: Kneel Ice Fisherman to take 1 gold token from any opponent's gold pool,and add it to your gold pool.`" or the agenda Kings of winter " " If it is Winter,any opponent with as many cards in hand as you,or more,must randomly discard a card from his or her hand at the end of the draw phase. If it is Summer,any opponent with fewer cards in hand than you draws a card at the end of the draw phase,if able."
What I don`t know that in a game, when it is winter, and when it is summer? Because I found not a word from it in the rulebook.