This is a very lightweight mod that uses WoW's built-in Equipment Manager to automatically equip a specific set of gear whenever you enter a town, and re-equip whatever you were wearing before whenever you leave. It does this by creating two new gear sets: Evening Wear and Last Equipped.
Usage is simple. Once you've installed the addon and logged into the game, simply change your Evening Wear gear set using the built-in Equipment Manager to whatever you'd like to wear around town. The addon should automatically detect whenever your rested state changes, and swap your gear accordingly.