Avocados ripen after they are harvested, triggered by ethylene gas. You can use this to your advantage.
The Paper Bag Trick
Place the avocado in a brown paper bag with a banana or an apple. These fruits release high amounts of ethylene, which gets trapped in the bag and speeds up the avocado.
The Window Sill
Warmth also helps. A sunny window sill can shave a day off the ripening time.