Having worked in retail management, Christmas always started earlier than November...my last SM job was at the large dollar chain. The month of September we had the following seasons displayed...
Leftover summer toys
Leftover back to school
Thanksgiving packaways from the previous year
Christmas packaways and clearance
It was always a time of the year that we, as managers pulled our hair out! Hallmark does their big ornament reveal in stores in July...I was usually done with Christmas music by the second week of December, but now that I'm out of retail, I can listen even after the 25th. Traditionally, for me, it was black Friday that kicks it all off. It IS getting earlier every year!
For me, Christmas starts the day after Halloween. We don't put up our tree until closer to Thanksgiving, but I love the excitement of the holiday season and creating family memories with all the Christmas festivities!
I would venture to suggest October. I just said to my wife yesterday that we better get to our local Walmart this week to pick up pool chemicals because in a few weeks the pool stuff will be off the shelves and the Christmas decorations will be on the shelves. The Hallmark Channel starts showing Christmas movies every day right up until New Years starting in July.