Yeast will normally be stored in a refrigerator until you are ready to use it. Dry yeast does not require being stored in a refrigerator as it is freeze dried, but if you want to be on the safe side, feel free to store it in the fridge. Liquid yeast always needs to be kept in a refrigerator.
White Labs remains viable in the fridge for 3 - 4 months after production. Wyeast will tend to store a little longer at 6 months. Keep in mind that for every month the yeast is kept in the fridge, you want to pull it out a day earlier. If it has been in the fridge for 3 months, then pull it out 3 days before use. 5 days before hand would be the earliest that you would need to pull yeast out.
The big question becomes what do I do with a yeast that has past its expiration date? We would recommend making a starter several days ahead of time, and seeing what the yeast does. It will likely come back to life and because of the starter you will have plenty of yeast cells to make your beer. We have used liquid yeast that was 2 years after the expiration date and had no problems because we made a starter. Fresh is always best, but sometimes you gotta do what you gotta do.