Launched in 1992 as an after-hours system to complement the pits, Globex gradually took everything. It runs roughly 23 hours a day, five days a week, with a daily maintenance break, and it is why futures are the most accessible overnight market for US traders.
Globex also defines the rules retail traders live under: the matching-algorithm, price-banding, velocity-logic pauses and the session boundaries that determine rth versus eth.
Example: ES trades on Globex from 6:00pm Eastern Sunday to 5:00pm Friday, pausing 5:00-6:00pm each weekday. The overnight session sets the tone before the US cash open.
Related: rth, eth, price-banding, cme-group