The plan is to stagger the release dates over several months and the 18-core Extreme Edition will be the last to arrive.
Intel said you'll be able to buy the lower-end 4-core, 8-core, and 10-core Core i9s as soon as next week. The mid-range 12-core Core i9-7920X will arrive in August.
The fastest of the Core i9 family will go on sale in October. The 14-core Core i9-7940X, the 16-Core Core i9-7960X, and the 18-core Core i9-7980XE Extreme Edition will be available in October or November.
If you are thinking it is all rather slow, you are probably right. While it puts everything in place for the Christmas sales, it will be hard to find anyone who will remember to put 18-core Core i9-7980XE Extreme Edition in their letter to Santa.
AMD will be rubbing its paws with glee as Intel's roadmap will give it a much needed headstart for its 16-core Ryzen Threadripper which is now more certain to arrive earlier more than later than summer. AMD's 16-core Threadripper now seems more likely to dominate CPUs in multi-threaded tasks, although we are sure that a large number of people will disagree with us in the comments.