Whitsunday

[hwit-suhn-dey, -dee, wit-; hwit-suh n-dey, wit-] /ˈʰwɪtˈsʌn deɪ, -di, ˈwɪt-; ˈʰwɪt sənˌdeɪ, ˈwɪt-/
noun
1.
the seventh Sunday after Easter, celebrated as a festival in commemoration of the descent of the Holy Spirit on the day of Pentecost.
Origin
before 1100; Middle English whitsonenday, Old English Hwīta Sunnandæg white Sunday; probably so called because the newly baptized wore white robes on that day
British Dictionary definitions for Whitsunday

Whitsunday

/ˌhwɪtˈsʌndɪ; ˌwɪt-/
noun
1.
(in Scotland) May 15, one of the four quarter days
Word Origin and History for Whitsunday

"Pentecost," late Old English Hwita Sunnandæg "white Sunday," possibly from the white baptismal robes worn by newly baptized Christians on this day.