Fans are divided on the issue. One popular suggestion is that Varys is descended from the female line of the Blackfyre Pretenders and has been working to put either a Blackfyre Spoiler (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. 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). 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.