Day traders are after action. Their success comes only in the face of market volatility, and wide range movements in individual stocks. During periods of inactivity a day trader is forced to choose his trading time very carefully. This leads them to look for the highest odds times to trade, but more importantly, the highest volume times to trade. The time zone that allows for the best odds of success.
As a day trader, the opportunities for trades are pretty focused these days. Most day traders look for stocks that are gapping higher due to earnings, or news events. This allows the greatest reaction from all participants, hence higher volume and more volatility. This is great during earnings season which is a major holiday for US day traders. The issue is that this earnings season is just that, a temporary moment in time. During the off earnings season day traders still need actionable ideas.
Scouring the news looking for stocks that are gapping higher or lower due to news events is the next likely course of action. Thankfully, with technology these days traders can scan through thousands of stocks in just a few moments to hone in on the stocks they want to focus on. The goal, again being to find the names with the opportunity for the highest volume and volatility.
Lastly, with no other options the day trader is forced to look to trade the volume zone. This is not generally stock specific, rather a time of day where action can be at a higher level. This time occurs just minutes before the markets open and up to the first hour of the trading session. While this can be a minefield for the novice day trader, and is not really favored by the day trader, it is a "Plan C" that sometimes is the best plan.