Actually, Idris does understand the concept of a US Holidays calendar, as she has posted dozens of times in the thread you apparently haven't read, or you would know that.
There is no such thing as ANY US Holiday. The Federal Government holidays are holidays only for Federal government employees. Not for anyone else, unless their employers choose to make them so. Thus, a true US Holidays calendar would have nothing on it. Is that your point?
Easter is thus not a US holiday. In fact, it isn't even on the Federal Employee's calendar. Is that your point?
About 3 million Americans (meaning US CITIZENS) celebrate Holi. And millions more non-citizens residing in the US celebrate it. And now, thanks partly to this thread, many more know what Holi is.
If you are arguing that NEITHER Easter nor Holi belong on a US Holidays calendar, I'm with you. And, in fact, Apple's original US Holidays calendar did not include either. But that got a lot of people bent out of shape. So they put Easter back on. But Holi is just as real a holiday as Easter. As is Passover. And Eid al-Fitr. All of which are celebrated by Americans and are now on the calendar. I guess they could change the name of the calendar to "Some holidays that some people in the US celebrate, along with some non-holiday dates that are either useful or interesting." Then you could include April Fool's Day, which is on the misnamed (according to you) US Holidays calendar, the dates Daylight Savings Time starts and ends, the dates for the solstices and equinoxes, Halloween, Veterans Day, Columbus Day...