The day falls on the sixth anniversary of the 2003 truck bomb attack on the UN headquarters at the Canal Hotel in Baghdad, Iraq, which killed Sergio Vieira de Mella, the human rights chief, and 21 other people.
"The Canal Hotel bombing rocked the UN system to its core," Pillay said in a tribute to Vieira de Mello and the "selfless, often unrewarding and sometimes dangerous" work of aid staff.
According to the UN, attacks on aid workers have risen 61 per cent in the last three years, and the annual average is almost three times higher than it was nine years ago.
"We are targeted more and more," Elisabeth Byrs, a UN spokeswoman, said before Wednesday's event.
The increase is blamed partly on a growth in aid operations in the most lawless parts of the world, with Afghanistan, Darfur and Somalia accounting for almost two-thirds of all attacks.
But direct attacks by armed groups who consider aid workers to be on the wrong side of the conflict are also on the rise.
"The days are gone when driving in a white jeep with a humanitarian logo was a guarantee of safety," Jonathan Mitchell, the emergency response director for Care International, said.
"With governments and militaries increasingly using development work to win the 'hearts and minds' of people in conflict situations, aid workers have become the preferred and easy targets of militant groups looking to strike back."
Wednesday's event is being marked by a ceremony in Geneva, where heads of several humanitarian agencies will gather to pay tribute to those attacked.
It comes a day after two Afghan civilian employees of the UN assistance mission were killed in a suicide bomb attack on a Nato convoy.