Olive's army is here to stay

I have two olives trees and one has quite a lot of Olives – not enough for oil but certainly should be enough to cure and have with a G&T or three.
I know when they are picked (as in time of year) in warmer climates (e.g. France) but it’s cooler here. Does anyone know what’s the particular point/sign in the fruit that tells you it’s time to pick them?

(still running a day behind)

Elvis Costello