The Expansion Boost Special Event, advertised for Sept 3 through Sept 17 does not overtly seem to be active at the normal time (daily reset today). The blog post did not indicate a start time, which is atypical for blog posts.
Has the event (double mastery xp in HoT, S3, PoF and S4 maps) been cancelled? Will it start later? Confirmation of the start time would be very helpful.
(Hope it wasn't cancelled.)