...At least in Canada, and by sporadic adoption some other places.
The British Columbia Supreme Court ruled that modern polyamory doesn't violate Canadian anti-polygamy laws, and thus that polyamory is legal in Canada on November 23, 2011. The Canadian Polyamory Advocacy Association decided to use the anniversary of this ruling as a day of polyamory awareness and celebration, declaring November 23 "National Polyamory Day."
Because people enjoy having holidays and awareness days for the things they care about, folks in other countries have taken up the date as a useful day to rally around (if in a somewhat ad hoc manner). In the US it seems to get more attention on years where the day is further from Thanksgiving, but if you want to celebrate or raise awareness on the 23rd, know that it's a day meant to be shared and celebrated with loves, friends, and chosen family.
You can find the podcast at readyforpolyamory.fireside.fm, you can join us on facebook at www.facebook.com/groups/readyforpolyamory, follow on Twitter @lauracb88 & instagram @readyforpolyamory, on TikTok @readyforpolyamory, and if you'd like to support us financially we're on ko-fi at ko-fi.com/readyforpolyamory. You can find 1-1 and group peer support sessions here. The book is available on Amazon; please leave a review if you enjoy it!