I'm partial to the French Revolutionary Calendar.
From the beginning of the French Revolution (14 July 1789) the years of freedom were counted, mostly alongside the Gregorian year. On 22 September 1792, this era was replaced by the years of the republic, the first of which was the Gregorian year 1792. The second year of the republic was begun on 1 January 1793. But, the radical change in the French state was to be expressed by means of a new calendar, which led to the elaboration of a new calendar wholly independent of the Gregorian calendar.
On 5 October 1793, the French government decided to introduce the Revolutionary calendar. The era of this calendar was 22 September 1792, Gregorian. In France, a decimal system of units was created, and time should be measured in decimal units, too. Therefore, a year was divided into twelve months of three decades each. The remaining 5 or, in leap years 6 days, called jours complémentaires and declared feast days, were being placed at the end of the year. Days and months simply were to be numbered, which would have led to days like "the seventh day of the first month of the fifth year of the republic".
There was no fixed leap year pattern, but the year was begun with the day of the autumnal equinox, which was determined by observation at the location Paris. The period from one leap year to the succeeding leap year was called franciade.
Before putting the calendar in effect, a commission was given the task of checking it. The numbering of days and months was criticized, and the poet Fabre d'Eglantine created poetic names for the months according to the season in which they occur.
No. Name Meaning
1 Vendémiaire vintage month
2 Brumaire fog month
3 Frimaire sleet month
4 Nivôse snow month
5 Pluviôse rain month
6 Ventôse wind month
7 Germinal seed month
8 Floréal blossom month
9 Prairial pasture month
10 Messidor harvest month
11 Thermidor heat month
12 Fructidor fruit month
13 Sansculottides additional days
Within a decade the days were given names with respect to their position.
No. Name
1 primidi
2 duodi
3 tridi
4 quartidi
5 quintidi
6 sextidi
7 septidi
8 octidi
9 nonidi
10 décadi
This was abolished in 1806 when France reverted to the Gregorian calendar.