Weatherstock IX will be Saturday, July 22. Music starts at noon EDT and is scheduled to run to around 5 PM. Eighteen bands will perform. Check the schedule for pre and post activities:
http://the-strongfoots.net/lmb/ads/weatherstock9/WeatherstockIX-schedule-640.png
If you haven't seen it, this thread has great posters for the bands:
http://www.lotro.com/forums/showthread.php?654215-Landroval-Weatherstock-IX-July-2017!
The concert takes place on Weathertop in the Lone Lands. Last year, the official attendance was 788. It is a unique event. The much larger number of people logged in will affect performance, especially in the Lone Lands and as you approach Weathertop. If you are planning to attend, get there early because once the crowd arrives, it will be slower to move into and within the area.
Here is some advice to make it easier on your and other players' computers:
http://weatherstock.guildlaunch.com/forums/viewtopic.php?t=11646638&gid=316072
Like the Woodstock Festival that inspired this event, there will be issues, so come with an appreciation for the community and the effort it takes to put it on. Sadly, the event is sometimes disrupted by griefers who can cause the server to grind to a halt with certain skills. They get banned, but it may still require you to log in again, a slow process when hundreds are trying to do it at the same time.
An alternative to travelling to Weathertop is to watch in your browser on the official LOTRO stream:
http://www.twitch.tv/lotrostream
Weatherstock is a special, awesome event and definitely worth experiencing.