Q. We are adding Indigenous Peoples’ Day to our company calendar. Is the apostrophe appropriate, as with Presidents’ Day, or no apostrophe, as in Veterans Day?
A. According to CMOS 7.27, “Although terms denoting group ownership or participation sometimes appear without an apostrophe (i.e., as an attributive rather than a possessive noun), Chicago dispenses with the apostrophe only in proper names (often corporate names) that do not officially include one.” So, absent any officially sanctioned spelling for the holiday, we would write “Indigenous Peoples’ Day.”
If you compare named days that involve an irregular plural, you’ll see that it’s not a simple matter of possession versus attribution. There’s a Children’s Day and a Women’s Day and a Men’s Day—but try that without the possessive (Men Day?). Hence Chicago’s preference for the possessive.
As for “Presidents’ Day,” according to Title V, section 6103, of the United States Code (which covers federal holidays and is usually cited as 5 U.S.C. § 6103), that holiday is still officially Washington’s Birthday but has been expanded to honor other presidents. “Presidents’ Day” demonstrates Chicago style, legal name or not. Veterans Day is also named in Title V, without the apostrophe, and because it’s official, that’s how we style it when referring to the national holiday in the United States.
Which brings us back to Indigenous Peoples’ Day. If you’re following Chicago style, use the apostrophe. If you’re following Associated Press style (and the AP Stylebook), leave it out. But where the holiday is official (as it now is in many states in the US), follow whatever the official style might be. For example, in North Carolina it’s Indigenous Peoples’ Day, but in Maine it’s Indigenous Peoples Day (no apostrophe).