
A 2011 atheist holiday stylized Christmas tree, namely a Flying Spaghetti Monster (FSM) topped pine tree (Ѻ), the FSM god, i.e. the 2005 parody god of Bobby Henderson, replacing the traditional Christian “angel”, which in the Roman recension (50BC-400AD) replaced the “ba” (pharaoh-headed bird) of Osiris, representative of his soul about to ascend into the afterlife.

A 2011 atheist holiday stylized Christmas tree, namely a Flying Spaghetti Monster (FSM) topped pine tree (Ѻ), the FSM god, i.e. the 2005 parody god of Bobby Henderson, replacing the traditional Christian “angel”, which in the Roman recension (50BC-400AD) replaced the “ba” (pharaoh-headed bird) of Osiris, representative of his soul about to ascend into the afterlife.

In festivals, atheist holiday refers to any day, considered, from a secular point of view, “holy”, metaphorically speaking, or sacred, celebrated by a non-believer, atheist, free thinker, humanist or agnostic.

Newtonian Christmas | Dec 25 / Jan 4
Isaac Newton was born on Christmas Day, Dec 25, 1642, according to the Julian calendar (or Jan 4, 1643 in Gregorian calendar years), then in use in England. Non-believers such as Goethe, have spoken about Newton's birth as the "new Christmas"; for example:

“1642 [Newton ’s birth] is the Christmas of the modern age.”
— Johann Goethe (c.1810)

Recently, Richard Dawkins (2008) has spoken, similar to Goethe, of revamping Dec 25 to be celebrated NOT as the birth of Jesus Christ, who never existed (see: Jesus never existed), but as a celebration of the birth of Isaac Newton, who did exist. [4]

Thomas Paine Day | Jan 29
In the 1990s, the Truthseeker, an atheist periodical, began to call for the celebrations of Thomas Paine’s birthday, Jan 19, at “Freethinkers Day”. [1]

Darwin Day | Feb 12
Some secularists and atheists celebrate “Darwin Day” (Ѻ) on Feb 12 the birthday of Charles Darwin; the following are related quotes:

“When Darwin’s book hit the crowd, it was like Christmas for ex-Christians.”
— Jennifer Hecht (2003), Doubt: a History (pg. 405)

Ingersoll Day | Aug 11
Some atheists celebrate the Aug 11 birthday of Robert Ingersoll (1833-1899) as a holiday of sorts. [2]

Freethought Day | Oct 12
Some secularists and freethinkers observe Oct 12 as “Freethought Day”, an occasion that marks the anniversary of the effective end of the Salem Witch Trials. (Ѻ)

Christmas | Khoiak festival
The time of Christmas (Dec 25 to Jan 6), aka Khoiak festival (Dec 10 to Jan 8), as it was originally called in Egypt, prior to the Greek recension and Roman recension of Egyptian religion, with the traditional use of the Christmas tree, aka tamarisk tree (djed pillar or backbone of Osiris) as it was originally called in Egypt, and gifts, is a peculiar time for atheists; per general reason that that this is symbolic of the time that many great atheists have, traditionally been burned at the stake (with their books), for questioning the meaning of Christmas, according to its underlying religio-mythology components (see: religio-mythology scholars).

In the Old Testament (c.250BC), aka the Jewish recension of Egyptian religion, the Osiris cult, the religious belief system that gave rise to the origin of the Christmas tree, was forbidden; this is stated in Jeremiah 10:2-4 as follows:

The anti-Christmas minded women, at right, holding the sign "pagan idol", means that the Christmas tree is symbolic of Osiris and the ascent of his ba or soul, aka "angel" at top of tree, into the afterlife. In the Jewish recension, the Ra worship aspect of Egyptian religion was kept, while the Osiris worship was rejected. In the Roman recension, Ra worship was kept, aka belief in Ab-Ra-ham (Abraham) as patriarch, AND Osiris worship was re-added back into the belief mixture, in the form of Jesus Christ, aka an "Osiris anointed" belief system, Osiris being descendant from Ra (or Jesus being descendant from Abraham). All of this, of course, is lost to the modern ear, give or take a few in the know.

Christmas, for atheists, in short, is symbolic of what they have been "working their mind" (see: unlearn) to reject as untruth; the following are related quotes:

Quotes
The following are related quotes:

“I wanted to become an atheist, but I gave up. They have no holidays.”
— Henny Youngman (c.1990) [3]

“There is an assumption that atheists don’t ‘do Christmas’.”
— Elisabeth Cromwell (2011), “A Very Atheist Christmas” [1]

“All I want for Christmas is a rational view of a universe by physical laws.”
— Author (c.2014) (Ѻ)(Ѻ)

References
1. (a) Melton, Gordon J. (2011). Religious Celebrations: an Encyclopedia of Holidays, Festivals, Solemn Observations, and Spiritual Commemorations, Volume One (Thomas Paine Day, pg. 858; Ingersoll Day, pg. 425); Darwin Day, pg. #; Freethought Day, pg. #). ABC-CLIO.
(b) Thomas Paine Day – SecularSeasons.org.
(c) Ingersoll Day – SecularSeasons.org.
2. (a) Melton, Gordon J. (2011). Religious Celebrations: an Encyclopedia of Holidays, Festivals, Solemn Observations, and Spiritual Commemorations, Volume One (Ingersoll Day, pg. 425); Darwin Day, pg. #; Freethought Day, pg. #). ABC-CLIO.
(b) Ingersoll Day – SecularSeasons.org.
3. (a) Huberman, Jack. (2007). The Quotable Atheist: Ammunition for Nonbelievers, Political Junkies, Gadflies, and those Generally Hell-Bound (pg. #). Nation Books.
(b) Hecht, Jennifer M. (2003). Doubt: A History: The Great Doubters and Their Legacy of Innovation from Socrates and Jesus to Thomas (doubt quiz, pg. 466). HarperOne.
4. Judson, Olivia. (2008). “The 10 Days of Newton” (Ѻ), Opinionator, Dec 23.