Investopedia explains 'Expiration Time'
Since many public holders of options deal through brokers, they face different expiration times. The last day to trade an option is the third Friday of the expiration month, but the actual expiration time is not until the next day (Saturday). A public holder of an option usually must declare their notice to exercise by 5:00 p.m. on the Friday. This will allow the broker to notify the exchange by the actual expiration time on the expiration date. Furthermore it depends on the exchange, for example the Chicago Board of Trade limits trading on expiring options to 3:00 p.m. Eastern on the last trading day