I like to think of calendars as a canvas over which we create meaning in our lives. The moments along the cycle of the year which we mark as individuals, as communities, as cultures, even globally, paint a picture of where we find meaning in our experiences, memories, aspirations, and interactions. The way in which we craft our cycle of the year, from where we derive and express the meaning -- seasonal transitions, religious occasions, political events, sporting events, etc -- speaks to the nature of calendar as much as it speaks to the nature of the culture. In fact, the word culture and calendar both come from the same Proto-Indo-European root word: *kwelə, meaning 'to turn.'
Interestingly, in Biblical and Rabbinic Hebrew, there is no word for calendar. Of course ancient Israelites and their descendants were counting days. Modern Hebrew has terms for calendar, לוח שנה luah shanah, but not until the 19th century. The act of calendaring, of creating a definition and structure around the cycle of the year, is so innate and ingrained, the Torah even presents it as the first commandment given to the people while still in servitude. The first step towards actualizing liberation is to establish an independent calendaring system.
Ancestral time-keeping in the Jewish tradition has never been a passive experience. The calendar is not something that happens to you, it is something that you do in your life. For millennia, ancestral Jewish time-keeping was done by witness and testimony. People would watch for the new moon, which originally was not the absence of the moon as we refer to it, but the slightest sliver of the return of the moon which emerges around 18 hours after the full absence of the moon - this crescent is so faint that in the 21st century, pollution conceals its appearance in many places on the planet. The participatory nature of crafting the calendar was so important that the leadership of the people actively made the process more accessible in whatever way they could. Prohibitions around Shabbat were relaxed to accommodate testifying about the new moon, festive meals were served to witnesses, lodging was provided, and every witness had their moment with the court to offer their testimony. As new moons were observed and testimonies approved by the court, months of the year could be calculated and adjusted as needed in relationship with the cycles of the seasons of the year. By attaching different moments to mark transitions in the year with personal and communal ritual or gathering, we produce structures and symbols through which we can relate with the changing cycles of seasons around us and within us.
However, Jewish calendaring is not merely one cycle of time. The calendar itself contains many cycles which overlay and interact with one another. We have a cycle of festivals, a cycle of holidays, a cycle of farming, a cycle of ritual behaviors, a cycle of political events, a cycle of civic engagement, and so on. Culture is made up of behaviors, characteristics, and knowledge of particular groups of people. We most readily think of language, cuisine, music, speech patterns, etc. Yet, one of the most fundamental mechanisms and most useful tools we have in producing culture is in calendaring - an agreed upon system of counting days. Not as a linear representation of the passage of time, but as a cyclical experiential relationship with the process of time.
The foundational cycle of the Jewish calendar is rooted in the lunar relationship to seasonal harvest: the full moons preceding both equinoxes. Each cycle adds added depth and opportunities for meaning making. Due to historical circumstances of living under foreign rule and over an ever growing geographic distance, creating the calendar by observation became unreliable and in the 4th century a system of calculation replaced the system of observation. While the system of calculation removed the most active participatory aspect - the witnessing and making testimony of the phases of the moon - it retains the most essential aspects of being in relationship with the process of time.
In so many ways, being in relationship with ancestral Jewish time-keeping keeps us in relationship with the transmission of wisdom of ancestors. By collectively marking transitional moments, we gain a mechanism through which to observe the process of time through seeing ourselves in process. It is not by mistake or coincidence or happenstance that the very first collective mitzvah is to keep a calendar, because calendaring is perhaps the most fundamental and foundational means of cultivating culture that humans have.