con·grat·u·late / kənˈgrachəˌlāt; -ˈgrajə-/ • v. [tr.] give (someone) one's good wishes when something special or pleasant has happened to them: I went into the living room to congratulate Bill on his marriage. ∎  praise (someone) for a particular achievement: the operators are to be congratulated for the excellent service. ∎  (congratulate oneself) feel pride or satisfaction: she congratulated herself on her powers of deduction.DERIVATIVES: con·grat·u·la·tor / -ˌlātər/ n.con·grat·u·la·to·ry / -ləˌtôrē/ adj.

congratulate XVI. f. pp. stem of L. congrātulārī, f. CON- (intensive) + grātulārī manifest one's joy, f. grātus pleasing; see GRATEFUL, -ATE 3.
So congratulation XV.