Group Calendar 4 important things to know
Group Calendar 4 will not talk to any previous versions, including the betas of Group Calendar 4 itself. All guild/alliance members must upgrade before everyone will see and be able to sign up for all events again.
You MUST restart WoW to upgrade. Group Calendar will warn you if you try to upgrade in-game with a reloadui and will disable itself until you restart WoW.
Events you created will still be there when you upgrade, but all events from other players will be purged. They'll begin showing up again as those players upgrade to 4.0.
The User's Manual is no longed included in the addon and is available online instead at http://wobbleworks.com/groupcalendar/manual. It hasn't been fully updated for GC4 yet, but the majority of it is still relevant. I'll try to get it updated completely soon, but it's a low priority for now.
About Group Calendar

Group Calendar allows guilds or groups of friends to share an in-game event calendar. The guild can post raid events and members can post events for dungeons they need to do quests for. Additionally, players can provide feedback to the event's sponsor saying that they will or won't be attending an event.

Once installed Group Calendar replaces the minimap current time tooltip and sun & moon icon. This can be used to open the UI or it can be opened using the /calendar command.

For instructions on how to set up Group Calendar for your guild or for your personal use, see the User's Manual at http://wobbleworks.com/groupcalendar/manual

PS. Наверное не плохой аддончик