Whitsun: [OE] Whitsun is etymologically ‘white Sunday’. The name comes from the ancient tradition of clothing newly baptized people in white on the feast of Pentecost. The abbreviated form Whit began to be used with other days of the week (such as Whit Monday) in the 16th century, but its broader modern usage (as in Whit week, Whit bank holiday, etc) did not emerge until the end of the 19th century. => sunday, white[Whitsun etymology, Whitsun origin, 英语词源]
"Pentecost," late Old English Hwita Sunnandæg "white Sunday" (see white (adj.)); possibly so called from the white baptismal robes worn by newly baptized Christians on this day. Related: Whitsuntide.