Following up to "JennyC" :

Would not be very good for me though......I'm a 7 day atheist!

You could celebrate "Relativity day" which would be a holiday for the whole
family, or "Heisenberg's Friday" which despite it's name would be on a
different day each year. "Darwin Day" would start off being fixed but could
be changed and adapted by individual atheists to fit their timetable
better.
--
Tim C.