One of the biggest changes in Android 5.0 Lollipop is the new material design and in the latest Developer Preview available for the Nexus 5 and Nexus 7 (here is a guide on how to install it on the Nexus 4) most of the official apps have also gone through some mayor design changes as Google has implemented its new design guidelines in almost all of its apps. (full list of all Android 5.0 lollipop APK’s).
One of these apps is Calendar, the calendar App has always had the ability to change the background (wallpaper) of the app. These wallpapers have been redesigned in Lollipop, bellow is a full list of all the new redesigned Google Calendar Wallpapers. You can download a .zip with all the wallpapers here.
Source : Google Plus