The first way to answer this question is to address the “they.” The New York Stock Exchange and the NASDAQ are two separate entities, though both are stock markets and both are headquartered in New York City. On both exchanges, “opening” and “closing” bells are rung to signal the beginning and end of each trading day. The trading day starts at 9:30am EST and ends at 4:00pm EST.

The logic of who gets to ring these bells is a more interesting question. It’s basically pure public relations and promotional play to ring the opening or closing bell of the NYSE or NASDAQ. Lebron James might stop by to ring the bell because he is in New York playing with the Knicks, or it could be the CEO of Electronic Arts because his company is about to release the latest version of Rock Band. On some days, no one is scheduled to ring the opening or closing bells, and officials from the exchanges have to do the job themselves. Generally speaking, companies and individuals request to ring the bell and the individual exchanges choose who they will allow to do so.

It’s important to remember, though, that ringing the bell to start or end the trading day is a ceremony and has significance. You wouldn’t want to be remembered as the person who rang in the worst trading day of the year. On the other hand, if your company rings the bell on the opening or closing of the best trading day ever, it’s a victory for pretty much everyone!