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.