Bringing up Thanksgiving is a great point.
There are legitimate and non-trivial reasons to question the reasons behind nearly all of our national holidays, and why some groups might feel uncomfortable celebrating them. Columbus day is obvious, and his atrocities are extreme enough that I do think that’s one that should be phased out.
But let’s go through it: Washington’s Birthday/Lincoln’s Birthday/President’s Day - Washington owned slaves. Lincoln was insufficiently committed to freeing the slaves, and expressed racist sentiments.
Independence Day - Given that we appropriated the land from Native Americans, the commemoration whitewashes a historical crime. July 4, 1776 did not mean independence for hundreds of thousands of slaves. Some Native and African Americans might rightfully be loath to celebrate.
Memorial Day/Veteran’s Day - Some of the military actions in which US troops have taken part, historically, were just. Some of them were not, at least in the opinions of many. A blanket celebration of those who have served represents unreflective jingoism at its worst, and amounts to the further exploitation of veterans who may be viewed as victims of government misrepresentations and lack of social and economic privilege.
Thanksgiving - Represents a sanitized, self-congratulatory view of the relationship between settlers and Native peoples.
Labor Day - Two of the labor groups credited with spearheading the movement behind the holiday, The Knights of Labor and the AFL, tolerated segregation among their southern affiliates, and actively campaigned for the Chinese Exclusion Act and other nativist policies. They also had an iffy record on gender issues.
MLK Day - First of all, it says something that we don’t have “Malcolm X” day; part of the reason we’ve chosen to enshrine King is because he is a racial leader whose message was palatable to whites. But then there’s also King’s response to a teenager who wrote into a column King wrote asking for help dealing with his attraction to other boys. King’s response was compassionate, for his time - but he suggested that the boy see a psychiatrist, said that the boy’s feeling “was probably not an innate tendency, but something that has been culturally acquired,” and added encouragingly that “You are already on the right road to a solution, since you honestly recognize the problem and have a desire to solve it.”
These problems vary in severity, but none, I think, are trivial or frivolous points. There are counterarguments for each - but all of them would risk invalidating the perspective of a minority who felt excluded or marginalized by the celebration of days explicitly not designed for them and people who hated or disrespected core aspects of their identity.
A knee-jerk refusal to all reconsideration of our culture heroes and celebrations is not the answer. But neither is conceding to every objection that might be raised to the point where we have no one and nothing left to celebrate but people who reflect precisely our own views – at least until they become problematic as well.