Whit Sunday the seventh Sunday after Easter, a Christian festival commemorating the descent of the Holy Spirit at Pentecost (Acts 2). The name is recorded from Old English, in form Hwīta Sunnandǣg, literally ‘white Sunday’, probably with reference to the white robes of those newly baptized at Pentecost.

Whit Sunday, Whitsunday seventh Sunday after Easter Day, Pentecost. Late OE. Hwīta Sunnandæġ ‘White Sunday’, so named prob. from the ancient custom of wearing white robes by the newly baptized at the Feast of Pentecost. The shortening of the vowel of the first syll. obscured the composition of the word, so that it has been differently divided (whence Whitsun eve and week (XIII) beside Whit Monday (XVI), etc.).
Hence Whitsuntide Whitsunday and the days immediately following. XIII.