imo, event will start monday or tuesday of next week. it would be crazy to start on day of halloween b/c that's a busy day for most people and if something goes wrong it'll be wrong for the weekend. if they start after halloween, that's a joke. i think the forum updates (producer's letter, etc..) actually hurt their credibility. They have a tough time delivering on certain content and they definitely have a tough time with making dates. Who knows when the next expansion of levels will take place, if at all?
So, i wouldn't be surprised (as stated by a previous poster candylicks) if tombs were pushed back until after the winter event. There is not a lot of time between when this event goes live and when they need to complete and submit the winter content in december.
enjoy the halloween event people.