For the last three years Apple has held a September keynote on a Tuesday. Apple's September keynote will most likely (Not confirmed) be held on Tuesday September 8, 2015
But making the press travel on a holiday is considered "bad taste" and I'm sure Apple doesn't want to irritate the press. That's why a lot of speculation is showing that the 9th will be the keynote. Give the press Tuesday to travel so they don't have to leave and "work" on Labor Day, with the keynote Wednesday.