"The Sunday before Easter which introduces Holy Week. The distinctive ceremonies of the day are the blessing of the palms and, in the W[estern] Church and some E[astern] Churches, the procession, representing the Lord's triumphal entry into Jerusalem a week before the Resurrection. . . ."
The Oxford Dictionary of the Christian Church, 3rd rev. ed., ed. F. L. Cross and E. A. Livingstone (2005), s.v. "Palm Sunday," as reproduced in the e-reference edition accessed on 27 November 2011.