While "work out clothes friday" doesn't have quite the same ring as "fitness friday" does, the implication is different.
"Work out clothes friday" doesn't necessarily imply that any real "fitness" is required to participate, which sounds like fun, not work, and might mean that those employees who tend to be "fitness" adverse will be more likely to join in, which is the real goal of a program like this in the first place right?
Creating a healthy "community" in some workplaces is easy because it tends to create itself, as those who are healthy gravitate to each other. But how is that community nurtured and encouraged to grow within the workplace? An idea like this is great because it doesn't feel like "fitness" itself is what's being encouraged, its the idea of fitness. And everything starts with an idea.