Thanks to the astute eyes of reader Jadissa, we can report that Noblegarden is no longer present on the in-game calendar. This is likely a clear indicator that the in-game world event is getting pushed back. Based on what we know already (that it will be part of other achievements), it should not be assumed that the event was removed.
We've spoke about the upcoming changes to Noblegarden before: it might get delayed, and it will be part of the overall world event achievement What a Long, Strange Trip.
There has been no indication from Blizzard when exactly it will start up, but I wouldn't be surprised to see it go live towards the end of the month. We'll have to wait and see when this start date is, and we'll be sure to announce it here on WoW Insider as soon as we know.
This is not an April Fools joke.