Time of year plays into attacks?

The deadly explosions at the Boston Marathon on Monday were another reminder that sporting events remain attractive targets for terrorists.

But one expert tells FOXSports.com that dates worry those in the anti-terrorism community just as much as venues and popularity of events.

“It’s big day not only because of Patriots’ Day, but tax day,” said Eric Frost, co-director of San Diego State’s homeland security program.

Patriots’ Day — which commemorates a couple of early battles from the Revolutionary War and falls on the third Monday each April — is a state holiday in Massachusetts. In Boston and surrounding communities, it’s often a day to attend the Boston Marathon or head to Fenway Park for a Red Sox game.

The Red Sox — who traditionally host a game on Patriots’ Day — had just finished their series with the Tampa Bay Rays before the explosions.

Such dates typically lead to heightened level of alerts, but April 15 wasn’t even the most feared date this week — at least nationally. That’d be April 20. Saturday is the 14th anniversary of the Columbine High mass shootings. It’s also Adolph Hitler’s birthday, a date celebrated by white supremacists.

April 15 is also within a week of three other infamous moments of violence in America’s recent history that led to mass loss of life:

• The federal raid on the Branch Davidian complex in Waco, Texas, which led to the deaths of 76 men, women and children on April, 19, 1993;

• The bombing of the Alfred P. Murrah Federal Building in Oklahoma City, which claimed the lives of 168 people in the worst act of domestic terrorism in US history, was on April 19, 1995;

• The Virginia Tech shooting massacre, where one lone gunman killed 32 people before killing himself, was on April 16, 2007

“Everybody gets ready for 9/11,” Frost said. “April 20 can also be a time for people to make a statement."

And there can be no bigger statement — no matter the date — than attacking a major event or well-known location. Monday’s attacks were the deadliest since the 1996 Summer Olympic bombing that killed two people and injured 111.

The Olympic bombing was carried out by domestic terrorist Eric Rudolph, who said he planted the bomb to embarrass the US on a global stage.

That particular attack is of chief concern for those charged with keeping participants and spectators safe. It has become known as a “lone wolf” attack, a crime that is harder to halt by law enforcement and intelligence agencies because only one person carries out the planning and execution.

“We had the attacks of 9/11 where it was al Qaeda, an organized terrorist group,” NFL Director of Strategic Security Jeff Miller told FOXSports.com in 2011. “Now, if you look across the landscape you see the greatest threat to mass gatherings in the United States is the homegrown violent extremist. This is somebody who is off (law enforcement’s) grid and through the use of propaganda on the Internet is encouraged to act out.”

It was unclear Monday evening if one or more people were responsible for the Boston bombings. It was also immediately unknown whether the perpetrators were domestic or international in origin.

While the 1996 Centennial Olympic Park bombing was a reminder terror can hit sports, it was the terror attacks of Sept. 11, 2001, that was the wake-up call.

“It just made everyone in the security community aware of the risks that are out there in regards to intentional events, including at sporting events,” said Christine Gibbs Springer, a professor of crisis and emergency management at the University of Nevada at Las Vegas.

And maybe the Boston Marathon could have been a target no matter where it fell on the calendar.

“It was a large gathering and global audience, which is a time of opportunity irrespective of the day,” Frost said.