No, you can post in the funhouse now! It simply means that when the new community opens, you won't be able to comment to the day one post because your character won't have arrived there yet. You can either have your character explore the funhouse on the first day but not be able to find the entrance to the new area until they come back to the funhouse on the second day, or you can have them find the entrance on the first day in Mayfield but have the funhouse itself warp time so that he arrives on the second day in the new area.