If they follow the same pattern as last year the party dates will be announced early to mid July. I just looked up when I bought my ticket for last year's Halloween Parties and it was July 29, the first day they were available to the general public.
I'll be there in early October this year again as well so I'm hoping they'll announce them a little sooner this year. Mostly because I'm impatient and want to be able to fully plan out my trip!