I've actually designed a few, with pen and ink drawings and little quotes.
But I just wanted to acknowledge the Gregorian one, which is being honored today over on GeneaBloggers.
On 24 February 1582, Pope Gregory XIII, enlisting the expertise of
distinguished astronomers and mathematicians, issued a bull correcting
the Julian calendar, which was then 10 days in error. The correction was
a minor one, changing the rule about leap years. The new calendar named
for him, the Gregorian calendar, became effective 4 October 1582, in
most Catholic countries, in 1752 in Britain and the American colonies,
in 1918 in Russia and in 1923 in Greece. It is the most widely used
calendar in the world today.
This still was trying to become corrected back to a birthday for Jesus, which was not exactly when the calendar began. But it does work better for leap years, and solstices and equinoxes, a bit.
I always laugh about how there were no babies born (in Catholic countries) on the dates Oct 5-14, in 1582. Genealolgists pay attention to those little details, and many have changed dates of events based on that 1752 adoption of the Gregorian calendar in Britain and the Americas. That must have made a bit of difference for lots of our ancestors' birth and death dates, around 10 days difference based on the change in 1582. But I've seldom seen any changes of that magnitude. No answer to that puzzle.