Hass avocados start as smooth, bright green fruit. As they mature, their skin turns increasingly pebbly and purplish-black. The longer the fruit is left on the tree, the higher the oil content and richer the flavor it will develop.
The best way to tell whether your fruit is ready for harvest is to pick one nice, large, dark avocado from your tree, and then leave it out on the counter at room temperature. At this point, the thing is rock hard.
If the fruit softens evenly within a week or two, then the rest of your avocados are good for the pickin’. If the fruit turns rubbery or shrivels up, then it isn’t quite ready yet.
You can speed up the ripening process by placing your avocados in a paper bag, which helps them ripen sooner by trapping the natural off-gas that they generate (an active plant hormone called ethylene).
If you want to speed it up even more, you can stick an apple or banana inside the bag (or any other ethylene-producing fruit, such as peppers, peaches, tomatoes… whatever you have on hand).