New Delhi, Oct. 3 -- Robinhood Markets is pushing US stock trading closer to a 24/7 model, raising a bigger question for investors: when does one trading day actually end and the next one begin?

The retail brokerage plans to let customers trade a selected list of stocks and exchange-traded funds over the weekend, in what would be a first for modern US equity markets. The service will run from Sunday at 8 p.m. New York time through Friday at 8 p.m., with weekend orders routed to a trading venue operated by Bruce Markets.

"Breaking news doesn't wait for an opening bell," said Steve Quirk, Robinhood's chief brokerage officer.

But longer trading hours do not necessarily mean that the traditional trading day has disappeared.

"The extension...