The Cycle

Unlike the day/night cycle however, the seasonal cycle is not completely based upon and dependant on the real world seasonal cycle. Instead, each season lasts a month. This cycle allows for all four seasons to be done within four months, following which, the cycle gets repeated. This means that you will have 3 opportunities to access each season in each year.

Spring Jan, May, Sept	Summer Feb, Jun, Oct	Autumn Mar, Jul, Nov	Winter Apr, Aug, Dec

Effects

The Seasons have several effects upon the game. First, is the obvious aesthetic change. Each season has a slightly different look, depending on what part of Unova you're in. In addition to that, the weather will sometimes change. Normally, it may rain in one area, but it'll snow in that area in the Winter.
In addition to that, there are a few core changes. First, the wild Pokémon in each area will vary significantly from season to season. For example, some Pokémon will be common in the Summer, but rare in the Winter or even exclusive to a specific season. This only occurs in Route 6, Route 7, Route 8, Route 20, Twist Mountain, Icirrus City, Moor of Icirrus, Dragonspiral Tower and Undella Bay
Secondly, several areas will have new areas accessible within certain seasons. An example of this is just above Icirrus City where, in Winter, a snowfall allows you access to a cliffside previously unaccessible. This only occurs in Winter and in the areas of Icirrus City, Route 8, Moor of Icirrus, the Twist Mountain and Dragonspiral Tower. In addition to this, in Route 20 during Autumn, various leaves will fall, giving access to many new items and Castelia Sewers empties of water in Autumn and Winter

In addition to that, the background music will change in a variety of areas to reflect the season. Such as in Winter, you'll hear more bells in most areas and Undella Town's music is more upbeat during Summer. The final effect is the change in the way the time of day works. In previous games, the times that the different parts of the day was static, but here, like in real life, the seasons affect what time each part of the day is.

In addition to this, the Royal Unova leaves each day at varying times.

Encounters & Other Changes

There are some season specific events that run throughout the game.
Firstly, in Undella Town, Cynthia will battle you each day in Spring, but be in th villa in Spring and Summer, being visited by the female Gym Leaders and Elite Four members of Unova.
In Icirrus City, the Shadow Triad will rechallenge you every month
In N's Castle, you can battle N once a season