"The feast of Harvest, called 'feast of Weeks' in Ex 34:22; it was held seven weeks, Dt 16:9, or fifty days, Lv 23:16, after the Passover (hence its Greek name 'Pentecost', Tb 2:1) and marked the end of the wheat harvest; at a later date it also commemorated the giving of the Law on Sinai."
Exodus 23 note d.2.
And, we must add, the descent of the Holy Spirit!
Here are a couple of suggested Christian greetings for this Octave.
[Filius] mittit Spiritum sanctum. Alleluia!
V. Loquebantur variis linguis Apostoli, Alleluia!
R. Magnalia Dei, Alleluia!
Lauds, None, and Vespers, Octave of Pentecost, and Alleluia verse of Feria Secunda infra Octavam Pentecostes. (EF)
P.S. Though the Pentecost Octave is suppressed in the OF, I am using the Pentecost sequence (which is in our missalettes, hurrah!) as a post communion meditation; challenging but very moving with its shadowy melody.