It's not hard to understand why secular leftists don't feel at home in America today. Consider the major holidays we celebrate: New Years' Day, President's Day, Good Friday and Easter, Memorial Day, Fourth of July, Labor Day, Thanksgiving, and Christmas. Out of all of these there are only two in which a Leftist can feel comfortable: New Years' Day, a secular holiday, and Labor Day, a time to honor the nation's workers.
All the rest must be a source of irritation and alienation for someone who despises the successes of capitalism and who disdains Christian belief. Surely, socialists and materialists will not care to join in honoring dead white males like George Washington and Abraham Lincoln on Presidents' Day. Washington was a slave owner and Lincoln forced the nation to fight a war that took half a million lives. Nor do people of the secular left care to observe Memorial Day which honors American soldiers, a species many on the Left regard with barely concealed contempt. Fourth of July is especially unpleasant, full as it is of the detestable trappings of patriotism and "My country right or wrong" sentimentality.
The remaining holidays are celebrations for those lobotimized, uneducated "Christers" who seek to force their silly superstitions on the rest of the country. In the ideal secular society they would not even exist. Thanksgiving could be a decent enough holiday from a socialist perspective if the idea were to give thanks to the government for its manifold blessings, but it's a little inane to be giving our thanks to Someone who's not out there to receive them.
All in all, the calendar must be a source of dismay and annoyance for those who see nothing much worth celebrating, either in the history or the religion of this country. They would be far happier, presumably, if instead of honoring Washington, Lincoln, and God incarnate we instead celebrated the birthdays of Marx, Darwin, Nietzsche, and Freud.
They could, perhaps, lump them all into a single observance and call it "Screw Up the World" Day.