Interesting to know, avocados do not ripen on the tree. Maybe for this reason we often find them still hard at the markets. This is by no means that they are an inferior fruit. An avocado is ripe when its deep green skin turns deeper in colour and gives slightly when gently pressed.
Unripened avocados should not be refrigerated. Just allow them to ripen in the fruit bowl along with apples or bananas. You can hasten this process by placing them in brown paper bag at room temperature.