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Rome's calendar came to dominate the ancient world at the points of the swords and spears of Rome's legendary legions.  [[Julius Caesar]]'s reforms of that calendar made it one of the most useful of the ancient calendars; [[Pope Gregory IX]] would reform the calendar further more than a millennium and a half later. [[Muhammad]], the great general and "[[prophet]]" of the Arab world, invented a calendar of his own and spread that throughout the Middle East at the points of the scimitars of his own armies. China has always had a calendar of its own and has never totally abandoned it.

Rome's calendar came to dominate the ancient world at the points of the swords and spears of Rome's legendary legions.  [[Julius Caesar]]'s reforms of that calendar made it one of the most useful of the ancient calendars; [[Pope Gregory IX]] would reform the calendar further more than a millennium and a half later. [[Muhammad]], the great general and "[[prophet]]" of the Arab world, invented a calendar of his own and spread that throughout the Middle East at the points of the scimitars of his own armies. China has always had a calendar of its own and has never totally abandoned it.

In the eighteenth century AD, the [[France|French]] mathematician [[Joseph Lagrange]], as head of the French Revolutionary weights-and-measures committee, devised an observed solar calendar with months named for growing, harvesting, and climate seasons, not for Roman numbers (September, October, November, December) or dead Roman emperors (July, August) or gods (January, February, March, June, etc.). But he also abandoned the seven-day week of the Julian and Gregorian calendars in favor of a ten-day week (called a ''décade'' in French). Perhaps this feature more than any other made the calendar impractical. (The Biblical model is clearly a week of six days of labor and one day of rest; see {{Bible ref|book=Genesis|chap=1}}.) In any event Emperor Napoleon I switched back to the Gregorian calendar in 1806, probably to retain the favor of the Popes of Rome as he continued his attempts to conquer the world. A short-lived political movement called the Paris Commune would revive the [[French Republican Calendar]] toward the end of the nineteenth century but would then abandon it two years later.

In the eighteenth century AD, the [[France|French]] mathematician [[Joseph-Louis Lagrange]], as head of the French Revolutionary weights-and-measures committee, devised an observed solar calendar with months named for growing, harvesting, and climate seasons, not for Roman numbers (September, October, November, December) or dead Roman emperors (July, August) or gods (January, February, March, June, etc.). But he also abandoned the seven-day week of the Julian and Gregorian calendars in favor of a ten-day week (called a ''décade'' in French). Perhaps this feature more than any other made the calendar impractical. (The Biblical model is clearly a week of six days of labor and one day of rest; see {{Bible ref|book=Genesis|chap=1}}.) In any event Emperor Napoleon I switched back to the Gregorian calendar in 1806, probably to retain the favor of the Popes of Rome as he continued his attempts to conquer the world. A short-lived political movement called the Paris Commune would revive the [[French Republican Calendar]] toward the end of the nineteenth century but would then abandon it two years later.

In summary, calendars have tended to come into being as much for ideological reasons, or the vanity of any particular ruler, as for practical reasons. Likewise, they tend to fall out of popular use as much on account of military conquest as on account of the relative fidelity of any given calendar to the seasons or astronomical cycles it is supposed to chart.

In summary, calendars have tended to come into being as much for ideological reasons, or the vanity of any particular ruler, as for practical reasons. Likewise, they tend to fall out of popular use as much on account of military conquest as on account of the relative fidelity of any given calendar to the seasons or astronomical cycles it is supposed to chart.