How to choose avocados correctly is important. You won’t be able to mash them if they are unripe, but they go from perfectly ripe to past their prime in about 15 minutes. Unripe avocados can be purchased if you plan to use them later in the week. It is an exercise in futility to try to use them before they are ripe, though. I usually buy the Hass variety, which turn darker as they ripen (other varieties may not darken with ripeness). The color of this variety, in addition to the feel when you give them a gentle squeeze, will help you determine the level of ripeness. For immediate use, avocados should be firm but have some give when you squeeze them gently in the palm of your hand. Do not use fingertips when you squeeze—that can bruise them. If they have a lot of give, they are close to or past their prime. If they have no give, they will need a few days to ripen.