What changes each month: Pokémon, items, passages and Deerling forms.
In Unova, the season changes every month. It isn’t just scenery: some Pokémon appear only in winter, others disappear under the ice, passages open in the snow and some items can only be picked up at a certain time of year. Here is the complete calendar, so you miss nothing.
The season follows your console’s clock: there is nothing to do to trigger it. The full cycle lasts four months, and repeats three times a year. The dial below shows the season of the current month.