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.

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. This only occurs in Route 6, Route 7, Route 8, 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 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 certain 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.

Deerling & Sawsbuck

In addition to that, the seasons also have effects on specific Pokémon, most notable Deerling and Sawsbuck. Deerling is unique in that it has special forms that are dependant solely upon the season. Their forms change automatically when the season in-game changes when in your party so you can have multiple forms if you store it in the box. However, when you boot up the game with them in your party, they will all change. Unfortunately, there are no special effects for each form so it is a completely aesthetic change.

Seasonal Institute

In Route 6, there is a Seasonal Institute which is researching the effects of seasons upon Unova. One of them will ask you to show them each form of Deerling. If you do, he will give you a Leaf Stone.