In Bukkit there were a number of custom calendar plugins, that allowed you to build a custom date system. The Days of the week and the months could be named whatever you wanted. It was really great for rolplaying servers.
The one I used was just called RolePlayCalendar. I think there was another called MineCal. RPC had multiworld support and you could have a different calendar in each world.
Could a plugin like this be developed for Sponge? The original Bukkit mod can be found here!