iOS 9.3 is coming in the Spring and will introduce Night Shift as well as a number of other new features.
OS X Mavericks Now Available Free on Mac App Store
OS X Mavericks includes enhanced multiple-monitor support, big improvements to the Notification Center, and an improved Finder, along with better power management and improved performance in Safari. Other features such as the iCloud Keychain for saving passwords across iOS and Mac devices, new Mac apps for iBooks and Apple Maps, and UI redesigns for a number of built-in applications are now included.
OS X Mavericks is available as a free, one-step update for all Mac users running OS X Snow Leopard and above, and is a free download on the Mac App Store. [Direct Link]