The root of this comes from the old pre-computer market and pre-computer society in general, certainly before WWII and afterward up to around the 70's or so.
Wall Street, including the physical exchange itself, had a more seasonal pace to it, you could easily see and feel the difference; the pace and the atmosphere between a Tuesday in July and a Tuesday in October or March was night and day. Brokers and other business people associated with the market went on vacations with their families, by mutual consent everything just slowed down.
Enter the computer. 24/7/365. It knows no seasons, takes no holidays, needs no rest. It has changed trading and the street in general beyond recognition, and--perhaps unfortunately--there's no longer any month in which one can go away!