The Farmhouse orders will give you double experience points and country coins.

Please note that the bonus will already be included in the Farmhouse Orders that you'll receive during the Event! Any existing Farmhouse Orders that are stored in your Farmhouse - when the event starts - will not be doubled.

Once activated, the Harvester will function on the Mainland or Island too.
Source: Official Farmerama English Forums.

You will get 5 x EP when you harvest your trees and when you buy one XXL tree upgrade, you get a second XXL upgrade free!      Please note: the 5x EP concerns fruit-bearing trees, not decorative trees!
Source: Official Farmerama Forums.

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.

You will get 2x EP when harvesting.

Please check the event timer for happy tree day to be sure it’s already running before you buy.
Eventually refresh your screen when the event is supposed to have started, and check your News Page.