Calendrier

Before calculation became established, the fixing of the month was a matter of judicial and public procedure. The determination of the lengths of months and the intercalation of months were the prerogative of the Sanhedrin, at the head of which stood a patriarch; the entire Sanhedrin was not convened for this matter, the decision being left to a special court of three. The tractate Rosh Hashana of the Mishna preserves the procedure: witnesses came to declare that they had sighted the crescent moon. The witnesses who claimed to have seen the new moon the previous night came to give their testimony and were subjected to cross-examination; the members of the Sanhedrin were themselves versed in astronomy.

Once the month was sanctified, the information had to reach distant communities. Tradition recounts the lighting of fires on hilltops, relayed from crest to crest from the Mount of Olives to the Babylonian diaspora — a method that the Sages abandoned, according to the Mishna, when dissenting sectaries lit deceptive fires, replacing it with the sending of messengers. It was from this uncertainty of timing that the custom arose in the diaspora of a second day of festival. This device rested entirely on a central authority seated in the Land of Israel, whose legitimacy stemmed from rabbinical ordination. Its fragility already foreshadowed the crisis to come: what would become of sacred time if this tribunal were to disappear?

Roman pressure on Jewish institutions in Palestine in the 4th century made observation perilous. Under the reign of Constantius, the Sanhedrin was apparently prevented from inserting the intercalary month in spring; it placed it accordingly after the month of Av. Tradition attributes to the Patriarch Hillel II a decisive reform. In 358-359, the Patriarch Hillel II is said to have replaced the observation of the moon by witnesses with permanent mathematical calculation; the Jewish year can now last 353, 354, 355, 383, 384 or 385 days depending on the adjustments.

Recent historiography, notably championed by Sacha Stern in Palaces of Time, strongly qualifies this founding narrative [Stern, 2001]. The date of 358-359 derives from a late medieval source, and Stern demonstrates that the transition to a calculated calendar was a gradual process, extending over several centuries, rather than a single and dated decree. The mature mechanism rests on pillars of calculation: the average duration of the lunar month, the molad or mean conjunction, and the Metonic cycle of nineteen years distributing seven embolismic years. The first day of the calendar thus calculated corresponds to Monday, 7 October -3761 of the proleptic Julian calendar; this calculation yields a molad tohu, a "birth from nothingness," fixed to this date. Here, the memory of a founder and the archive of the scholar answer each other: the one embodies, the other distils.

The calendar was not merely a science but a matter of authority among the great centers of medieval Judaism. At the beginning of the 10th century, one of the most documented conflicts in the history of Jewish computation broke out. Aaron ben Meïr, gaon of the Palestinian Jewish community, attempted to reclaim a share of this authority in 921 by introducing a new triennial Jewish calendar that modified the date of Pessah and Rosh Hashana. The discrepancy, of a few hours in the calculation of the molad, resulted in the celebration of festivals on different days between the Land of Israel and Babylonia — a schism intolerable for the ritual unity of the people.

Facing Ben Meïr stood one of the greatest minds of his time. Rav Saadia Gaon ben Yossef (882-942) was born in Egypt; he attained the title of Gaon and directed the celebrated yeshiva. He lived in Babylonia from 882 to 942 under Muslim rule, and much of what we know of his work comes from letters and documents. The controversy reflected a deeper divergence of traditions. The controversy between Rav Saadia Gaon and Ben Meïr stemmed in fact from the difference in tradition between the Land of Israel and Babylonia. The victory of the Babylonian position, carried by Saadia, established the unified calculation that still governs the Jewish calendar today, and sealed the shift of the center of authority toward Babylonia.

Two centuries after the Babylonian dispute, Moses Maimonides (1138-1204) accomplished in his code Mishneh Torah the most systematic undertaking of calendar organization. In the treatise Hilkhot Kiddush ha-Hodesh — the Laws of the Sanctification of the Month — he expounds both the ancient procedure of observation by witnesses and the astronomy of permanent calculation. Maimonides' originality lay in integrating the astronomical models of his time, inherited from Greek and Arabic science, to rationally justify the determination of the visible crescent, while confirming the authority of the calculated calendar for the diaspora.

His exposition carefully distinguishes two regimes of time: the ideal one, of a restored Sanhedrin proceeding by judicial sanctification of the month, and the effective one, of fixed computation that supplements the absence of this tribunal. This architecture gave the calendar its definitive status as a halakhic discipline in its own right, taught and commented upon in all academies. Through Maimonides, calendar knowledge ceased to be an expertise reserved for a few Sanhedrin initiates and became a body of rules that was accessible, transmissible, and binding upon all.

Fixed by calculation, the calendar became the invisible skeleton of Jewish life throughout the Diaspora. It ordered the week around the Sabbath, distributed the great festivals — Passover in spring, Shavuot fifty days later, then Rosh Hashanah, Yom Kippur and Sukkot in autumn — and governed the cycles of Torah reading. In Sephardic communities of the Mediterranean basin as in Ashkenazi homes of Central and Eastern Europe, the same computation synchronized existences separated by thousands of kilometers.

This synchronization also had an economic and social dimension: commemorative fasts, days on which one could neither judge nor marry, periods of mourning between Passover and Shavuot rhythmed communal activity. Each new month begins at the new moon, the Molad in Hebrew. The sanctification of the new month, announced solemnly in the synagogue on the preceding Sabbath, kept alive, even in the most ordinary rituals, the memory of the vanished court of Jerusalem. The calendar was thus, more than any territory, the temporal homeland of a landless people.

In the twentieth century, the word "calendar" took on a tragically new meaning. In 1993, Serge Klarsfeld published The Calendar of the Persecution of the Jews of France, 1940-1944 [Klarsfeld, 1993], a monumental work of history founded on the administrative archives of the occupier and the French state. The work reverses the very principle of the computus: instead of marking out festivals and life, it enumerates day after day the anti-Jewish decrees, roundups, internments, and deportation convoys that struck the Jews of France under Vichy and German occupation.

This work illustrates a mutation in the meaning of the calendar: from a liturgical framework of sacred time, it becomes a grid for the memory of victims, a methodical instrument for reconstructing facts against forgetting and denial. Each date in it is backed by a documentary source — circulars, telegrams, lists of convoys — making it possible to establish the precise chronology of the crime. In this sense, Klarsfeld's work extends the deep vocation of the Jewish calendar: to make time a support for collective memory and a bulwark against erasure. The computus that once ordered the joy of festivals serves here to engrave, day by day, the name of those who were torn from common time.

Hillel II (4th century, precise dates unknown) — Patriarch of the Land of Israel, he is associated by medieval tradition with the establishment of the calculated calendar. He is credited, around 358-359, with replacing the observation of the moon by witnesses with a permanent mathematical calculation. Modern scholarship, however, considers this attribution a historiographical shortcut: the passage to computation was probably gradual. He remains the emblematic figure of the shift from observed time to calculated time, guaranteeing the ritual unity of a diaspora threatened by the disappearance of Palestinian institutions.

Aaron ben Meïr (10th century, dates unknown) — Gaon of the Jewish community of the Land of Israel. In 921 he attempted to reclaim a share of calendrical authority by introducing a new triennial calendar that modified the date of Pesach and Rosh Hashanah. His position, grounded in Palestinian tradition, provoked a temporary schism between communities. Defeated by Saadia Gaon, he embodies the last major attempt by the Land of Israel to maintain mastery of Jewish time in the face of Babylonian ascendancy.

Saadia Gaon (882-942) — Saadia ben Yossef al-Fayyoumi. Born in Egypt, he attained the title of Gaon and directed the famous yeshiva of Soura in Babylonia. Philosopher, grammarian, translator of the Bible into Arabic and polemicist, he successfully opposed Ben Meïr, imposing unified Babylonian calculation. He lived in Babylonia from 882 to 942 under Muslim rule. His authority permanently fixed the rules of the calendar still in use today.

Moses Maimonides (1138-1204) — Rambam, born in Cordova, died in Fustat in Egypt. Physician, philosopher and codifier of rabbinic law, he devoted in his Mishneh Torah the treatise Hilkhot Kiddush ha-Hodesh to the sanctification of the month. He united the judicial procedure of observation and the astronomy of permanent computation, drawing on the science of his time. His exposition made the calendar a rigorously codified halakhic discipline, transmitted in all subsequent academies.

Serge Klarsfeld (born in 1935) — Historian, lawyer and activist for the memory of the Shoah. He published in 1993 The Calendar of the Persecution of the Jews of France, 1940-1944 [Klarsfeld, 1993], a documented chronology day by day of anti-Jewish measures and deportations under the Occupation. His work transforms the very form of the calendar into an instrument for establishing facts and commemorating victims, in keeping with the memorial vocation of Jewish time.

Sacha Stern (born in the 20th century, dates of birth not precisely published) — Contemporary historian of ancient Judaism, author of Palaces of Time: Jewish Calendar and Culture in the Ancient World (2001). His work has profoundly renewed the understanding of the formation of the Hebrew calendar, showing that it results from a long evolution rather than a single, dated reform. He subjected traditional founding narratives to critical examination, notably the one attributed to Hillel II.

The history of the Jewish calendar is one of a slow conquest of temporal autonomy. Born from a court interrogating witnesses beneath the sky of Jerusalem, mastery of time shifted toward calculation, then toward codification, as the Jewish people lost their territorial institutions. Each stage — Mishnaic observation, the fixing attributed to Hillel II and disputed by scholarship, the dispute of 921, the synthesis of Maimonides — responds to the same imperative: to maintain the ritual unity of a people dispersed through a shared and common time.

In the twentieth century, with Klarsfeld, the calendar reveals an ultimate virtue: that of methodical Memory, capable of fixing through dates what barbarism sought to erase. From the sanctification of the month to the chronology of persecution, the Jewish calendar remains what it has always been: not a neutral measure of time, but an act of fidelity, a way of inscribing within duration the identity and Memory of a community.