Sorry if this is a noob question here, BUT how do you go from summer to winter? Is that a natural progression in the game, or does there need to be a card played?

I have alot of cards that do certin things in the summer/other in the winter. How do I know which is which?

The cards Black Raven and White Raven make it summer and winter respectively. There are three copies of Black Raven in each "A Song of Summer" chapter pack and three copies of White Raven in each "The Winds of Winter" chapter pack.

thank you kindly