Apple sent out invitations to the event on Tuesday.
Apple tends to send its iDevice media invitations about a week before the invent. Assuming this is indeed a media event for the iPad mini, we'd expect, based on past history, that pre-orders for the device would begin on Oct. 26.
The device would then reach retail a week later, on Nov. 2. It's possible since the event is being held on a Tuesday that Apple might rush the device to market and have it show up earlier, say as early as Oct. 26.
That seems doubtful, however, considering the planning that Apple invests into these product launches, that it would consider rushing "anything." In fact, it's even possible, though unlikely, that Apple would delay the date of the actual retail launch.
That's because of the fact that when Apple releases a "new product line," it usually extends the timeframe between the launch event and the release date (witness the extended periods between the iPhone and iPad events and release, for their first versions).
Despite the mini being a new product, we tend to think of it as an adjunct to the iPad line, thus not requiring the extended roll-out time. We'll know more on Oct. 23.