Do Brits really say "Whitsun Sunday"? Sounds a tad redundant...
On the contrary: although Pentecost is a religious feast, not a secular holiday, all Christians do observe it -- by no means only Catholics. And not just liturgical denominations either (notice where the Pentecostals got their name).
Maybe the calendar was designed in Hong Kong or somewhere? I think other US speakers will agree that "Whitsun" is exclusively British. Educated Americans who read know what it is, but most people on the street would have no clue.
umc & Anke:
Many Americans observe secular aspects of Easter (kids hunting eggs, eating chocolate rabbits, etc.), but Pentecost has no such corresponding cultural observances; so Easter is semi-secular, Pentecost wholly religious. Same with the calendar: although secular institutions don't take all of Holy Week as vacation (unlike, say, in Latin America), schools & offices here do sometimes close on Easter Monday, but never the day after Pentecost (unless it happens to be Memorial Day).
BTW, US readers also pause over UK seasonal descriptions like Michaelmas, whose original church use evidently carried over into things like school/university terms -- I still have to look that up to find out when it is, other than just sometime vaguely in the fall (I think).