These terms (from a Latin word meaning "to act," "to do") refer to something to be done. The former is singular; the latter, plural. Agendum is rarely used but occasionally is employed to refer to one item in a list or a program of action, to a single act or topic for discussion. Agenda applies to a list of items, such as a program for a meeting or discussion group. Although plural in form, agenda is a collective noun and is commonly used with a singular verb: "The agenda for this meeting has been set." A less-used form, agendums, requires a plural verb: "The agendums are lengthy and involved." Preferably use agenda (with a singular verb) or agendum if you wish to single out a particular item. See also addendum.
Dictionary of problem words and expressions. Harry Shaw. 1975.