"Elections will be held on the 26th of January," Dayananda Dissanayake, the commissioner of elections, told AFP by telephone.
Rajapakse, 64, called the election two years ahead of schedule in a move designed to help the government benefit from public goodwill after the end of the country's 37-year separatist conflict, which ended in May.
Local media had widely predicted the poll for January 23 or 26, which are considered astrologically auspicious for the president.
Fonseka, 58, is expected to confirm his candidature at a press conference on Sunday after he won the backing of Sri Lanka's two main opposition parties this week.
The former general quit as chief of defence staff this month, after a rift with Rajapakse over who should take credit for the military victory over the separatist Tamil Tiger rebels.
After the fighting ended, he was moved from the post of army chief to become chief of defence staff, a more ceremonial position that reportedly left him feeling sidelined