Effective Monday, September 21, the daily CME Globex and ClearPort maintenance period will begin 15 minutes earlier Monday through Thursday from 16:00CT/17:00ET until 16:45CT/17:45ET. Currently, the daily maintenance period occurs Monday through Thursday from 16:15CT/17:15ET to 16:45CT/17:45ET. With this change, the closing times for the following markets will now occur 15 minutes earlier Monday through Friday at 16:00CT/17:00ET. CME Equity, CBOT Equity, COMEX, NYMEX and DME. http://www.cmegroup.com/trading-hours.html
This is usually a conpletely garbage time to trade but I've never understood why these maintenance windows are needed on a daily basis. Surely almost everything is redundant and mirrored hence if data needs to be pulled or otherwise it shouldn't cause a disruption.