White Sunday. The seventh Sunday after Easter, to commemorate the “Descent of the Holy Ghost” on the day of Pentecost. In the Primitive Church the newly-baptised wore white from Easter to Pentecost, and were called albati (white-robed). The last of the Sundays, which was also the chief festival, was called emphatically Dominica in Albis (Sunday in White).
Another etymology is Wit or Wisdom Sunday, the day when the Apostlec were filled with wisdom by the Holy Ghost.
This day Wit-sonday is cald. For wisdom and wit serene fald, Was zonen to the Apostles as this day.
Cambr. Univer. MSS., Dd. i. I, p. 234.
“We ought to kepe this our Witsonday bicause the law of God was then of the Holy Wyght on Ghost deliured gostly vnto vs.” —Taverner (1540).
“This day is called Wytsonday because the Holy Ghost brought wytte and wysdom into Christis disciples ... and filled them full of ghostly wytte.” —In die Pentecostis (printed by Wynken de Worde).
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894