Just guessing but it seems there is not 5000 people to manually close trading on all 5000 stocks exactly at 16:00:00 instead it is a scheduled task on the computer which probably includes computer doing end of day things with actual shares and money which cause all kinds of havoc if done accidentally on a Sunday or a ..... Columbus day, someone probably forgot to update into some of the schedulers that Columbus day is a trading day this year and market close 'procedure' should be run. Good that there are still people there to handle these situations.