I know this site didn't host "server calendars", but it could. I also know they were for almost an opposite reason: When not to go to [Dynamis] rather than when to go.
Allow people to schedule events such as naakual, or even colo.reive parties. Perhaps even delve farming--but mostly public events. People could then perhaps try to be on and get the clear if they need it/gear if they get lucky.
Launch it, if possible, available on at least jp.ffxiah.com and might bring back a little bit of cross-language spirit.