These weeks are seven; they form a week of weeks, and their morrow is again a Sunday, the glorious feast of Pentecost.
These mysterious numbers – which God himself fixed when he instituted the first Pentecost after the first Pasch – were adopted by the Apostles when they regulated the Christian Easter
[...] Thus, then, the whole season of Easter is marked with the mystery expressed by each Sunday of the year.
Sunday is to us the great day of our week, because beautified with the splendour of our Lord’s Resurrection of which the creation of material light was but a type.
We have already said that this institution was prefigured in the Old Law, although the Jewish people were not in any way aware of it.
Their Pentecost fell on the fiftieth day after the Pasch; it was the morrow of the seven weeks.
Another figure of our Eastertide was the year of Jubilee, which God bade Moses prescribe to his people.
Each fiftieth year the houses and lands that had been alienated during the preceding forty-nine returned to their original owners; and those Israelites who had been compelled by poverty to sell themselves as slaves recovered their liberty.
This year, which was properly called the sabbatical year, was the sequel of the preceding seven weeks of years, and was thus the image of our eighth day, whereon the Son of Mary, by his Resurrection, redeemed us from the slavery of the tomb, and restored us to the inheritance of our immortality.
The rites peculiar to Eastertide, in the present discipline of the Church, are two: the unceasing repetition of the Alleluia, of which we have already spoken, and the colour of the vestments used for its two great solemnities, white for the first and red for the second.
White is appropriate to the Resurrection: it is the mystery of eternal light, which knows neither spot nor shadow; it is the mystery that produces in a faithful soul the sentiment of purity and joy.
Pentecost, which gives us the Holy Spirit, the ‘consuming Fire’ (Heb. xii 29), is symbolized by the red vestments, which express the mystery of the divine Paraclete coming down in the form of fiery tongues upon them that were assembled in the Cenacle.
Prosper Guéranger (1805-1875): The Liturgical Year, tr. Dom Laurence Shepherd, O.S.B., Vol. 6, (Newman Pr., Westminster, Md, 1952).