I know when I used to work in Woolies that as soon as the Back to School was over (so beginning of September), Halloween would start to come in (along with the odd bit of Christmas!) There would be a little bit to start with, one bay or two, then towards the end of September/beginning of October, it would start to expand.
Thankfully, I bought all the left over Halloween stuff last year, so have an entire bag full! haha But I don't know when or where I'll get stuff when this starts to run out!