Christmas season for retailers in the US is Black Friday (the Friday after Thanksgiving, which is the fourth Thursday in November) through Christmas Day. A lot of retailers have started doing sales the entire week of Thanksgiving, basically early Black Friday sales.
Most stores start receiving Christmas goods in October to set up for the Christmas season, although like many mentioned above, it is sometimes much before October. It takes time to set everything up, so to the consumer it might look like retailers are starting ridiculously early.
Some stores do set up ridiculously early, but I rarely see anything before September, unless you are in Frankenmuth where it is Christmas all year. https://www.frankenmuth.org/things-to-do/christmas/ There are several stores and towns that are Christmas all year around in the US. Some were mentioned above by other jellies.
Other Christmas things start early November, like Nutcracker might play in your town before Black Friday.