"The election is on 17 November," the island's elections secretariat said on Monday.
A De Silva, assistant to Elections Commissioner Dayananda Dissanayakesaid, said the commission would invite formal nominations of candidates by 17 October.
Outgoing President Chandrika Kumaratunga's second and final
six-year term ends on 23 December.
Rajapakse, who is popular with the Sinhalese Buddhist majority and has aligned himself with hardline Marxists and monks who detest the separatist Tamil Tigers, is fervently against privatisation and has vowed to nurture local business to take on foreign competitors.
Business leaders and investors are betting on a win by Wickremesinghe, who is perceived as more market friendly and more likely to reach a permanent peace deal with the Tigers.
But many worry he is out of touch with the middle class and poor, who voted him out of power in 2004.
Poll analysts say the vote revolves around two key issues of how to cement peace with the separatists and manage the economy.