The Season Unending only takes place if you haven't progressed far enough in the Civil War. If not, you will be tasked with negotiating a truce between the two sides of the war so that you can use Dragonreach to capture a Dragon. Talk to the Jarl of Whiterun and he will either have you seak out the leaders of each side for talks or he will grant you permission to use his Castle. If the latter, you can skip to the Fallen.
Talk to each leader and then talk to the Greybeards about using their place for the meeting. Once everybody gets together, the negotiations will begin and you will have to make choices about who gets what territory and so forth. These decisions CAN change how the war plays out when you do that quest line. It won't be anything to major but it's neat. You really can't make any wrong choices so choose how you see fit. Once the negotiations are done, head back to White run.