To forbid Christians from celebrating Easter on the first day of Passover, Roman Emperor Constantius prevented the rabbis of the Land of Israel from proclaiming new moons and leap years based on observation of the heavens. This information had been sent to the Jewish community in Babylonia but was now forbidden.In the year 358 CE, nasi (known by the Romans as “patriarch”) Hillel II once and for all standardized the calendar and sent it in writing to the rabbis in the Diaspora. While occasionally the gaonite (presidents of Talmud academies) in Babylonia disputed the date of holidays set by the nasi in Israel, Hillel II’s calendar has endured and is the basis of the Hebrew calendar today. While Hillel II’s work has seemed to set in stone the holidays of the Hebrew calendar, there is some fluidity in his work. The only addition accepted by all Jews was the celebration of Simhat Torah, about 1,000 years ago. In addition, Religious Zionists have accepted Israel Independence Day as a legitimate holiday. Despite the standardization of the holidays by the nasi, the importance of certain holidays has changed over time. The importance of certain holidays is fluid – I think of Hanukkah and Lag Ba’omer. This fluidity is certainly the case of the 15th of Shvat, in Hebrew the holiday of Tu BiShvat. What did Tu BiShvat celebrate for 2000 years? The Mishna, edited by Judah HaNasi in 200, refers to the Fifteenth Day of the Hebrew month of Shvat as “the New Year for Trees.” Why did trees need this New Year? I turn to Rabbi Irving “Yitz” Greenberg’s The Jewish Way: Living the Holidays (1988) to answer that question: “There is no trace of the festival in the Bible. The origins of the day may lie in the ancient custom of celebrating the first day of each season. A Talmudic passage describes the year as divided into six seasons. From 15 Shvat to 15 Nissan is the season of kor (cold) which comes after the season of choref (winter) and before the season of katzir (reaping, harvesting).” 15 Shvat was a special day in an agricultural, rural society.The occasion emerged as an important day on the Hebrew calendar 500 years ago. The great mystics and legal minds exiled from Spain in 1492 found a home in the Galilean city of Tzfat (Safed). They adapted the Passover Haggadah to Tu Bishvat. But instead of liberation, the focus was on Kabbalah – the celebration of the bounty of the Land of Israel was given mystical meaning as an ushering in of the Tikkun (the reunification of the Godhead, not “Social Justice”) and the coming of the Messiah.This Tu Bishvat seder has gained popularity since its creation and has achieved more importance since the Jewish return to Israel in modern times. While these rabbis were certainly waiting for the Messiah, rituals such as the Tu Bishvat seder established them as proto-Zionists in their yearning for redemption in a living land, not an abstraction of “The Jerusalem of the Heavens.”The modern movement of the Jewish people to the Land of Israel moved what was a Kabbalistic holiday to a national one. The 15th of Shvat gained even more importance in the immigrant experience and psyche. In a chapter on “The Religious Motifs of the Labor Movement” penned by Anita Shapira in Zionism and Religion (1998), the historian states: “Another holiday thus transformed was Tu Bishvat: In the Diaspora it was dedicated to the seven kinds of fruit that blessed Palestine, expressing the longing of a people for a different country, with different fruits and climate. In its new guise it became the holiday of tree planting, in keeping with the scriptural injunction, ‘When you come to the land, you will plant there a tree.’ This children’s holiday was, perhaps, the most ‘Zionist’ one of all, symbolizing the rejuvenation of the land. Thus, a day of little significance in the rabbinic tradition became a major festival today as “Israel’s Arbor Day.”Shapira’s assessment bolsters the idea that the Hebrew calendar is fluid. Hanukkah, a minor festival of post-Torah origin, becomes the celebration of Jewish sovereignty in the Land of Israel. The miracle of the oil in the modern mind takes a back seat to the Maccabees’ founding of an empire after triumphing over a major power. Lag Ba’omer, traditionally associated with the end of the plague of the students of Rabbi Akiva, is transformed into a celebration of the last sovereign Jewish state in the Land of Israel before 1948, before the beginning of a long exile. The rabbis were critical of Bar Kokhba’s messianic revolt and derided it until the reality that Jews would return as a sovereign entity in the Land of Israel. The Zionist movement turned history on its head, transforming minor holidays into major celebrations. In fact, the celebration of planting trees on Tu Bishvat is not that far removed from the ancient acknowledgment of the coming of spring in an agrarian society. If Judaism is to be a vibrant and relevant religion, the calendar must be fluid and “minor” holidays should be elevated in importance. The elevation of Tu Bishvat, Hanukkah, and Lag Ba’omer should embolden us to celebrate Israel Independence Day, Jerusalem Day, and religiously commemorate Yom Hashoah. Times change – we have to change with them.The writer is rabbi of Congregation Ashei Sholom in West Palm Beach, Florida.