The President's Day Massacre was a large-scale terrorist attack orchestrated by the Sons of Jacob and marks the earliest stage of the Republic of Gilead's founding. The attack is shown to have occurred in both the TV series and the original novel, though it is only referred to by this name in the latter.
The specific date of the attack is unknown, but it is confirmed that it took place in Washington, DC and targeted the highest echelons of the US government. Specifically, the attackers succeeded in bombing the White House and machine gunning the US Capitol building, resulting in the deaths of the (unnamed) US President and most of the US Congress. Plans for a similar attack on the US Supreme Court are mentioned by Fred Waterford and Serena Joy in a conversation that takes place in the attack's planning stages, though the specifics of that attack have yet to be clarified on-screen. Following the massacre, the Sons of Jacob seized control of the US government and armed forces through unspecified means, declaring martial law, suspending the US Constitution and gradually reshaping American society into the "utopia" of Gilead.. According to the original book, the Sons of Jacob were able to blame the attack on Islamic terrorists, a detail which has not yet been explicitly mentioned in the TV series.
In the television series, a still of a newspaper prop from the second season reveals that the attacks took place in September 2014 in Washington, D.C.. The date of the publication is listed as September 15, 2014; this means that the attacks took place on September 14 or in the days before. Coincidentally, this date is very close to the actual 9/11 attacks in 2001 carried out by Al-Qaeda militants. This contrasts with the date of the real President's Day holiday, which occurs on the third Monday of February.
The still documents the immediate shock and reaction by the remaining United States government and citizens. The president's death was the result of fatal gunshot wounds to both the chest and shoulder with his place of death being Georgetown University Hospital, meaning that it wasn't the bomb set off at the White House that caused his demise. It is revealed that it wasn't just elected officials who were attacked, but also regular people. The headline calls the attack "America's bloodiest day," implying a death toll exceeding that of the 9/11 attacks, which in turn implies that the great majority of the victims were civilians. People belonging to many religious denominations, as well as atheists, were said to be joining condolence gatherings around the country for prayer and comfort.
Ironically, the Sons of Jacob, who orchestrated the attacks, are said to have raised millions of dollars with the intention of donating the said amount to the families of the individuals that they covertly massacred while also calling for "empathy and unity".
The country, as described in the novel ("a suspended state of animation"), is said to not have a government (though whatever is left will continue to function); martial law in effect; and with much suspense and little details regarding the attacks.