This mini event will give 2 x EP when you harvest your stables (not Pastures) vand when you buy one Classic II stable (or Orange Upgrade), you get a second identical stable free! Please note: The second Orange Stable upgrade only applies if Orange upgrade is purchased from the Supply Store; not an Orange upgrades built in the Workshop.
This 'bonus EP x 2' will work also for your Production Farms, and Spooky Stables.
Source: Official Farmerama Forums.