Despite having gone twice during the Halloween season (although never for MHP sadly) I'm not sure if all the Halloween decorations are up for the first weekend of Halloween season in September or do they add more through the month?
mummabear --
Yes -- everything will be in place by the first official day of Halloween Time (except MHP-specific things), and maybe earlier.
The official "Halloween Time" season, as it is now known, didn't actually begin with that name/title until 2006 (according to the historical dates I looked up online and posted towards the end of the previous Superthread that I found), so things were probably done a bit differently during the year of your first Halloween trip in 2005 -- meaning that the decorations were likely not out and on display by a hard start date in mid-September at that point. Haunted Mansion Holiday has been around since 2001, so those HMH-specific decorations would have been around on whichever date HMH opened in 2005.
As
Mary484 said (by the way, welcome,
Mary!

), some of the holiday season decorations do, indeed, appear before October 31st -- which I referenced in Post #1/Page 1 of this thread and in the Christmas/Holiday Season Superthread this way: "
There is even a rather fun 'colliding holidays'/crossover period of time in which pumpkins still dot the Disneyland landscape while wreaths and garland slowly move in."
This is a necessity, as the holiday season is a major, all-encompassing, resort-wide undertaking at DLR and the decorations process has to start early or it would never be done on time.
I was told by a CM last year that DLR tries to handle the holiday decorations transition in a way where that certain things make sense. In other words, they're most likely not going to place Christmas trees side by side with pumpkins, but the snow appearing on the Castle is something that can signal the changing of the seasons and weather in certain spots. In some places around the world, it will be snowing in October!
I think that, at one point, some of the New Orleans Square decorations were appearing before Halloween as well. I don't know if that still happens, but the masks seem fitting to me to be out and on display before Halloween.
That said, some of the Halloween things begin to appear in August! There have been reports of Halloween merchandise, in-store displays and window displays popping up in August. Some of the seasonal goodies begin to appear early too.
As for the more obvious Halloween decorations, the above-turnstile pumpkins appear earlier in the week before Halloween Time begins. So let's say Halloween Time begins on Friday, 9/12 this year -- and I don't know that it will, but let's use that as a date for the purposes of my example. If 9/12 were the start date, the turnstile character pumpkins would probably appear on 9/8, if not earlier than that!
The giant Mickey pumpkin that holds court in Town Square seems to appear within a day or two of the Halloween Time season start date.
The Halloween Carnival/Roundup begins to get its overlay a bit early, though the Villains probably wouldn't be there before the Halloween Time start date.
I'm not sure how early the pumpkins in the windows and on balconies along Main Street begin to go up.
In any event, all of the Halloween Time decor and events will be in effect on the start date of the season -- except for anything involved in the MHP!