If you want to ripen an avocado overnight or in a shorter amount of time you can place the unripe avocado in some plastic wrap and then place the wrapped avocado into a brown paper bag.
The brown paper bag and the plastic wrap traps the ethylene gas that is released from the avocado which helps to ripen the avocado faster.
Then place a banana, apple, or tomato into the brown paper bag which also helps the unripe avocado ripen.
Close and seal the paper bag and let it sit overnight and in the morning the avocado should be ripe but if not let it sit again for another hour or two and the avocado should be ripe by then.