The event started first on October 11, 2016, and ended on November 1, 2016.
In 2017 the Halloween Terror Event started on October 10th and lasted until November 1st.
In 2018 the Halloween Terror Event started on October 9th and lasted until October 31st.
In 2019 the Halloween Terror Event started on October 15th and lasted until November 4th.
In 2020 the Halloween Terror Event started on October 13th and lasted until November 3rd.
It is a safe assumption, but only an assumption at this time. There have been no official word to when the Halloween Terror seasonal event will return.
Saturday or Sunday afternoon you should start seeing skin previews with an event teaser leading up to Tuesday showing 2-3 skins. So we’ll know more in a few days. God only knows what awful marketing people they have working there that always try to make annual events a pseudo-surprise instead of hyping them up for 7+ days before the event starts like any other game with annual events does. Oh well…
I wish they did let people know sooner what the skins were. We know theres no new events as they would’ve talked about that awhile ago. I know they are putting time into OW2 but they could at least do something simple for a new event for one of these seasons. (I came out of a year of not posting just to put this out there haha).
Hyping the skins would bring back a decent amount of players- at least for a decent amount of the season (as we also have the weekly based skins that come out to keep people active per week)
Need:
Evil Nurse Mercy
Terminator 2 Reinhardt
Quasimodo Torb (so I can shoot him a lot!)
Skeleton Baptiste
and
Magtheridon Orisa
I think this makes sense but if Blizzard spreads out their releases they might change it up a bit? Hearthstone Mercenaries releases on the 12th as well, so we’ll see.
Possibility Tuesday 12 .
same i think tuesday next week or i hope it is. maybe we will see a reveal of the skins over the weekend or 1 of the skins
I hope so. There’re other games have their Halloween events started (e.g Killing Floor 2).
I’m very hungry for a teaser / preview.
I say 19th or maybe even 21
They’ve been a bit inconsistent the past whole year about event reveal. Some events they announce a week before with a skin/cosmetic reveal a day leading up to the event.
Some of the events we get the announcement a day before it goes live with a skin or two reveal the same day, the it goes live. And some events get announced the day before going live and no reveal.
I am banking on it going live next Tuesday as it lines up well with the 3 week length of October 12 - November 2.
The National Day of Spain 
I was hoping for an event reveal today with a skin reveal for it but I haven’t seen anything. Maybe tomorrow?
It might bring me back. I mean, I will probably come back regardless for Halloween. But the skins will be the difference of whether I log in 3 times to get the free weekly boxes and play Junkenstein exclusively. Or if I actually come “back” back.