After four years of negotiations in Havana, Santos, 65, and Timochenko, a nom de guerre for the 57-year-old revolutionary, will shake hands for the first time on Colombian soil in front of world leaders.
Their deal to end Latin America's longest-running conflict will turn the FARC guerrilla group into a political party fighting at the ballot box instead of the battlefield it has occupied since 1964.
Some 2,500 foreign and local dignitaries will attend the ceremony in the colonial city of Cartagena, where huge billboards call on Colombians to accept the peace plan.
Guests include UN head Ban Ki-moon, Cuban President Raul Castro, US Secretary of State John Kerry and victims of the conflict.
