Humanist Holidays – Dec 23 is Human Light

Here is a short list of various days and events which have been developed by humanists around the world.  The IHEU endorses World Humanist Day (21 June), Darwin Day (12 February), Human Rights Day (10 December) and HumanLight (23 December) as official days of Humanist celebration, though none are yet a public holiday.
DARWIN DAY, Feb 12
WORLD HUMANIST DAY,  June 21
HUMAN RIGHTS DAY,  Dec 10
HUMANLIGHT,   Dec. 23

Humanists may also recognize other dates, such as
HYPATIA DAY, March 15 A pagan, and probably an atheist, Hypatia of Alexandria was a woman of remarkable intellect who advanced mathematics and the science of astronomy in her time. Her death at the hands of a christian mob in March 415ce has been described as marking the end of classical antiquity.
“All formal dogmatic religions are fallacious and must never be accepted by self-respecting persons as final.”  —attributed to Hypatia, unverified
EARTH DAY,  April 22
PI DAY, March 14
Pi Day is an annual celebration commemorating the mathematical constant π (pi). Pi Day is observed on March 14 (or 3/14 in the U.S. month/day date format), since 3, 1, and 4 are the three most significant digits of π in the decimal form. In 2009, the United States House of Representatives supported the designation of Pi Day.^[2]
Pi Approximation Day is observed on July 22 (or 22/7 in the day/month date format), since the fraction ²²⁄₇ is a common approximation of π.

But for something truly encompassing, see
CARL SAGAN’s COSMIC CALENDAR  (illustrations at this link)
Cosmic Calendar is a scale in which the 13.8 billion year lifetime of the universe is mapped onto a single year. At this scale the Big Bang took place on January 1 at midnight, and the current time is mapped to December 31 at midnight.  At this scale, there are 434 years per second, 1.57 million years per hour, and 37.7 million years per day. The concept was popularized by Carl Sagan in his book The Dragons of Eden and on his television series Cosmos as a way to conceptualize the vast amounts of time in the history of the universe.