Usually the expression "good old" or "good ol' " is used to express an action that you've done or experienced a lot, and the familiarity of it brings comfort. Usually it's said when referring to the action in a fond or positive manner, but it can be said sarcastically as well. "Good old leg day" probably refers to the day in which you pay special attention to working out your legs as opposed to other body parts. The person saying it would either be saying that they enjoy leg day, or if they're being sarcastic, they could be dreading the day. It would depend on the context in which it is said to figure out which usage is being applied.