What is the earliest I can buy tickets for October? They do not have dates available of the calender

They do have availability in October. I would recommend checking the website you are on. When I was purchasing online, it took me a few tries to find the actual website link and not through Groupon or TripAdvisor. They show limited dates. Go here instead https://fareharbor.com/embeds/book/thehauntedmuseum/items/61939/calendar/2019/01/?full-items=yes

Best way to buy tickes would be at Museum on day you attend