To me in September, given that they said act 5.4 would be delivered by the end of the year. It would make no sense to release 5.3 in October and 5.4 in December. Moreover, 6* will be introduced as defenders in October and they also stated there won't be 6*s in 5.3 so it would make sense that 5.3 would be released in September.
Comments
It'll be twice as hard as 5.2 and create 10x as many discussions about why it's "not fair".