We've discussed this at length in several of the past threads, but FWIW once again ...
The word Whitsun, and the use of it for school terms and the like, was only traditional BE and is largely unknown in AE.
Pentecost is certainly known to religiously literate AE speakers who attend or grew up in mainline churches, and that demographic group is still a somewhat larger proportion of the US population as a whole than in the UK or Europe, even though there's a greater dropoff now among the youngest generations.
The biblical event in Acts is surely also familiar to some evangelical churchgoers, though a significant number of younger evangelicals seem to be people who come to shopping-center-style churches for the rock music, the big video screens, the basketball gyms, the twelve-step groups, etc., but may not actually be very biblically literate.
But the traditional liturgical calendar is often hardly followed in many evangelical or nondenominational US churches at all, except for Christmas and Easter. (And ironically, perhaps especially not in charismatic aka Pentecostal congregations? No idea, sorry.
Robert -- US might have known.) So it's entirely possible that many nonliturgical US Christians don't observe the Sunday of Pentecost, or even know when it is.
And as for public holidays, the US does not observe Easter Monday or the week after Easter
, much less Pentecost or the week after Pentecost
. So in order to be clear, you would pretty much need to write out those phrases, and in the case of the week after Pentecost probably also add the actual date, as only a minority would have the remotest idea.
As in the UK, we do have a public secular holiday that falls roughly around the same time, though it's not the same thing. In our case it's Memorial Day, the last Monday in May, which makes a three-day weekend that marks the approximate beginning of summer.https://en.wikipedia.org/wiki/Memorial_Day