Well it isn't exactly an bug more like an mistake that hasn't been fixed. The calendar is open for those who aren't logged in - but for those who are logged in it isn't possible to access it.
That's a standard setting on all TribalWars forums. There is a reason for this - there are events visible to those logged in that are not visible to those that are not. That's why registered accounts cannot use it (unless you are a moderator). Moderators put vacations on there and other events. Its not a bug but could be changed later.
