Calendar has it's icons in several places.
Find Calendar in the Applications folder. Right-click it and choose Show Package Contents. Open the Contents folder, and then the Resources folder. Here you find the two icons called App-empty.icns and App.icns. Copy them to somewhere as backup or rename them. Copy the replacement icons App-empty.icns and App.icns into this Resources folder, choose to Replace the current icons, and authenticate your action. Now go in the Resources folder to a folder called Calendar.docktileplugin and open it. Choose the Contents folder and then the Resources folder. Copy the App-empty.icns into this location also, replacing the older icon. Finally, open Terminal and type in killall Dock to restart the Dock. The new icon should be applied!