Avocados are an unusual fruit. Unlike fruits such as apples and oranges, avocados do not ripen on the tree. Instead, they only ripen fully after they have fallen. Fortunately, avocados do not ripen at a set pace once they have separated from the tree. You can speed the ripening process by putting them in a paper bag with an apple, for example. While you cannot completely stop the ripening process indefinitely, you can also dramatically slow the rate at which avocados ripen.
Purchase the hardest and lightest green avocados you can find if you want to have ripe avocados later rather than sooner. If you have already purchased the avocados or grew them yourself, select the hardest and lightest green avocados from your collection.
Place the avocados you wish to stop from ripening loose in the refrigerator. However, if your refrigerator has a tendency to freeze in some parts, such as against the back or sides, do not place the avocados in these sections.
Refrigerate your avocados for up to five days. The ripening process will be very slow during this period, and the avocados you remove from the refrigerator should look very similar to the ones you put in days before.
Remove your avocados from the refrigerator three to five days before you are ready to use or serve them. Place them in a warm area to allow them to ripen properly. If you have made a mistake in estimating how long to leave them in the refrigerator, you can speed the ripening process by placing the avocados in a brown paper bag with an apple or a ripe banana.
AvacadosFromMexico.com says you can refrigerate ripe avocados at 36 to 40 degrees until ready for use for up to three days
References and ResourcesEUFIC: How Can One Speed the Ripening of an Avocado?
Avocados From Mexico: Knowing When an Avocado Is Ripe
Avocados From Mexico: Ripening and Storing Your Avocado From Mexico