As mentioned the holloween content and from my understanding any content like this is always added permanently. To trigger it you have to go to tattoo parlour during a full moon to hear Odette and girls talk about the graveyard, then go to the graveyard that night or on any other nights during a fullmoon to be able to trigger the halloween scene.
At the moment it's unknown how to know when a full moon will occur though you can talk to Odette after having seen the scene about the graveyard and your able to ask about when the next full moon is by picking to talk about the graveyard. She will then respond with either "the last full moon was a few days ago, the next one is in a couple of weeks", "the next full moon is just over a week away", "the next full moon is on *random day*".
In short first response means it will be weeks before the next full moon, second response it's about a week left, last one will tell you which day has a full moon for instance, next full moon is on tuesday.