How can I tell if an avocado is ripe?

Avocados do not ripen or soften on the tree. They ripen after they have been harvested. Depending on the season, avocados and other green fruit require  2-7 days to ripen at room temperature. To check for ripeness, gently squeeze the fruit in the palm of your hand. When avocados yield to gentle pressure, they are ready to eat.    
 
If you don't intend to use the avocados immediately, look for specimens that are hard and don't yield when they are squeezed.        
 
Source(s):
Avocadocentral "Frequently Asked Questions"
Avocadocentral "Preparation Tips and Techniques"