You are absolutely right and I am glad those days are over, as it states in this link, I got from a Google search of your phrase. Thank you. Relieves a concern.
“ From 1950 to around 2013, the Dow Jones Industrial Average posted lower returns during the May to October period, compared with the November to April period. Since 2013, statistics suggest this seasonal pattern may not be the case anymore, and those who follow it may miss out on significant stock market gains.”
Perhaps online brokers trades started in 2012 - 2013