(possibly Aegon, if he is a fake as some have suggested)
or a Targaryen (as the next best thing) back on the Iron Throne. Illyrio is working with him as payback for all the help Varys gave him during his rise to power.
What is a year?
In our world, at ASoIaF's approximate level of social and scientific progress, a year was simply one Summer to the next, or one Winter, one Christmas, whatever. But they don't have that pattern to work from. So how come everyone seems to know how old they are, how many years ago things happened, etc? How do they keep the record?
The planet has a moon which orbits it once a month. Twelve of those orbits equal a year, which by stunning coincidence equals one of our years :)
Another suggestion is that the world had 'normal' weather up until the Others' invasion thousands of years ago, and that event upset the seasons and cause them to go haywire. The length of the year may be a tradition held over since then. This may also tie in to the extremely loose grasp of history in Westeros (since we've been told since AGoT that the Andal invasion was 6,000 years ago, but in ADWD we're told it could have been 4,000 or even 2,000, since the tracking of the years and seasons was extremely erratic, even after the arrival of the Andals and their stronger record-keeping).
Do the days still get longer and shorter, even if there is no winter or summer?
No. The days get longer and shorter in keeping with the length of the seasons. So a summer that lasts for four years will have long summer days throughout the entire period (one suggestion has been that the maesters keep a careful track on how long each day has lasted, and once the days start getting shorter again, even if it's barely perceptible, they know the season has turned and can start warning people to stockpile food for the winter). It's this element that makes scientific explanations for the erratic seasons completely invalid, as there is no way to explain how the days change in such a fashion.
I believe that the world of Westeros is a planet larger, but less dense, than Earth and orbits at the exteme edge of the habitable zone for their star. Their solar system is a double sun system, with a red dwarf on an eccentric orbit. At one point, a character (Jon?) is commented on as watching the red wanderer crossing the night time sky.
Originally Posted by Vooloc
To me, this answers the question of why there are long, but unpredictable winters/summers. When the red dwarf is near enough, it's heat maintains the warm weather of Summer. As it recedes, the lack of a secondary heat source returns the planet to a cold world status. Toss in axial tilt, regular summer/winter orbital effects, and you end up with a world where you never know if winter will bring white walkers.