Discussion Groups / English Usage / January 2007

athel_cb@yahoo.co.uk had it:

By being thought of as a ceremony marking the beginning of one's
status as a doctor or master.

The OED says "esp. at Cambridge, Dublin, and the American
universities".  They cite it to 1387, with respect to "the universite
of Oxenford", although a 1714 quote says that the term is by then used
at Cambridge, but not Oxford.

   1387 TREVISA _Higden_ (Rolls) VI. 259 By a statute of the
        universite of Oxenford..he schal not spende at his
        comencement passynge þre þowsand of grootes turonens.

   1587 HARRISON _England_ II. iii. (1877) I. 75 In Oxford this
        solemnitie is called an Act, but in Cambridge they vse the
        French word Commensement.

   1593 NASHE _Four Lett. Confut._ 74 Shewe mee the Vniuersities hand
        and seale that thou art a Doctour sealed and deliuered in the
        presence of a whole Commensement.

   1689 _Lond. Gaz._ No. 2496/2 (Cambridge) An extraordinary
        Commencement being held on this signal Occasion, for
        conferring Degrees on persons of Worth in all Faculties.

1714-23 AYLIFFE _Univ. of Oxf._ II. III. i. 131 There is a general
        Commencement once every Year in all the Faculties of
        Learning, which is called the Act at Oxford, and the
        Commencement at Cambridge.

   1858 MASSON Milton I. 163 Three days before the close of the
        academic year..there was held at Cambridge the great public
        ceremony of the 'Commencement'.

   1890 Academy 5 July 12/2 Dublin University..The recipients of
        honorary degrees at the commencement are, etc.