M. Paul Bryant-Quinn wrote
> `Triduum' refers to the three days of a particular feast celebrated,
> with reference to the period from Good Friday to Easter Sunday.
The Oxford Dictionary of the Christian Church (usually a good starting
point) entry reads:
Triduum Sacrum (Lat., 'the sacred three days'). The three concluding days
of Holy Week commemorating the Last Supper, Passion, and Death of Christ,
i.e. Maundy Thursday (evening only, acc. to modern RC reckoning), Good
Friday, and Holy Saturday.