With all the info for ARR I have yet to see any kind of in game calender/event planner feature. Most current gen MMOs have some type of in game calender that players can use to plan in game events. I played MMOs that have this and it makes organizing raids a breeze.

With a ingame calender you could:

-Create events, invite people to a raid on a day and time, allow them to accept or decline. If you invite the same group of people a lot have copy, and paste features to quickly make new raid events. Meet someone had a good time with them you could easily add them to an later event you are doing. Very helpful for organizing a raid and seeing who can make it and who can't.

-Create linkshell/free company events with a sign up feature. A leader or officer can create events for people in their linkshell to sign up for on the calender.

-Show when lock outs to certain raids will expire.

-Devs can post stuff on the calender like when special in game events occur like holiday events, GC appreation or what have you.

-Lodestone / cell phone app, Intergration. If someone is not currently in the game they could log in the lodestone click a events tab to see if their LS is planning any events or if anyone else in the game has invited them to a raid or event or sent them a message. When they respond on the website or through the app it is reflected in real time on the in game calender.