Over the next few weeks, a host of new items and challenges will arrive, including an in-game Challenge to earn the new Crossbow weapon, more remixed game modes, new playlists, and more in the Store to discover. To make room for all that activity, we are making a one-time extension of Season One its now ending on February 11. During that time, there will be 2XP, 2X Weapon XP, and 2X Tier Progression to help you finish up your Officer Ranks and Tiers, and get you ready for an exciting Season Two.
Source: N4G PC Call of Duty: Modern Warfare Season One Extended