Bungie has quietly confirmed that there will not be a Festival of the Lost Halloween-themed event in Destiny 2 this year.
Responding to an inquiry on Twitter, community manager DeeJ said that the developer was too focused on the game launch and subsequent rollout of key activities.
Festival of the Lost saw players given Halloween-themed quests that provided candy as a reward, which we could then spend on various Halloween loot bundles, usually including paper masks that let us dress up as key Destiny villains and characters.
Sounds like Festival of the Lost or something like it will probably return next year, if you're still playing, but in the meantime you'll just have to do fancy dress the old way - find some clothes that look like something in pop culture and demand treats from old people.