"Although there are actually several matches, most people know the NRMP as the Match. . .The basic modus operandi of the Match is as follows: After the interview season, residency training programs submit a list of applicants in the order in which they would offer acceptances, and students enter lists of programs in the order in which they would accept offers. Both students and programs then submit their ranksing to the NRMP. Subsequently, in a matter of minutes, a computer program matches each student to the highest program on his or her list that offered a position. The Match usually occurs in late February and its results are announced simultaneously across the country on "Match Day" in mid-March."
--taken from First Aid for the Match.