Whitsun, or hvítasunna in Icelandic, is a religious holiday, celebrated on the seventh Sunday after Easter. Usually known as Pentecost in English, the holiday commemorates the descent of the Holy Spirit upon the disciples of Christ.
In Iceland it is common for children to be confirmed on Whitsun, with Easter being another important date for confirmations. It is a coming-of-age ceremony, when 14-year-olds confirm their faith in God before entering adulthood.
According to Wikipedia, the name ‘Whitsun’ is a contraction of ‘White Sunday,’ attested in “The Holy-Ghost, which thou did send on Whit-Sunday” in the Old English homilies, and parallel to the mention of hwitmonedei in the early 13th-century Ancrene Riwle.
Walter William Skeat noted that the Anglo-Saxon word also appears in Icelandic hvitasunnu-dagur, but that in English the feast was always called Pentecoste until after the Norman Conquest, when white (hwitte) began to be confused with wit.
The name derives from the white garments worn by catechumens, those expecting to be baptized on that Sunday, when infant baptism was still uncommon.
Thus it is centuries older than the tradition of the young women of the parish all coming to church or chapel in new white dresses on that day.