Obama will be the second former U.S. president to deliver the lecture—the first was Bill Clinton in 2003, when the annual lecture was inaugurated. Others to deliver the speech have included Microsoft founder Bill Gates, superstar economist Thomas Piketty, and anti-apartheid struggle veteran Desmond Tutu.

Obama’s speech will be particularly significant, coming on the day before the centenary of Mandela’s birth. Mandela, who was South Africa’s first black president and even before that an international icon for his struggle against the racist apartheid system, died in 2013—Obama gave a lengthy address at his memorial.

Those giving the Nelson Mandela lectures are supposed to “drive debate on significant social issues…in order to address the challenges we face today,” according to the Nelson Mandela Foundation.

The big question is whether, a day after President Donald Trump’s widely panned summit with Russian President Vladimir Putin—in which Trump opted to believe Putin’s denial of Russian tampering with the 2016 election over that of his own Justice Department and intelligence services—Obama will make any reference to his successor.

According to Obama aide Ben Rhodes, speaking to Associated Press, Obama will not refer to Trump directly. However, some may read a Trump reference into Rhodes’s explanation of Obama’s theme: “At the current moment we’re in, values that we thought were well-established—the importance of human rights, respect for diversity—in many parts of the world those values are under threat.”

Whatever he says, Obama has not given many public speeches since he stepped down as president, so many people will be listening.