There are a lot of wonderful events that happen every weekend in St. Helena.  Many wineries offer special things such as "crushing today"  where you will get to see them crush the grapes, or bottling or barrel samples.  Most have sandwich boards along the road so you can see what is going on.    Because you can't really plan in advance when sugars will rise and who will pick what vineyard when, you generally don't know until you get here.  September and October is what is called Crush or Harvest, when wineries throughout the Valley are picking grapes.  It varies depending on varietal, weather, location etc.  That is why it generally covers a 16 - 24 week span.

 A couple of good places to check on events throughout the Valley is on http://www.napavalley.com/events.html

and http://www.sthelena.com/events.html