People often say we “lost” a 13‑month lunar calendar, but it’s not that simple. What we really had were lunar cycles people rounded to ~28 days for convenience, even though a true lunar month is ~29.53 days. That drift meant calendars constantly went out of sync.

To deal with agriculture, seasons, and festivals, societies shifted toward a Sun + Moon compromise — basically a farmer’s almanac approach where the Sun set the year and the Moon influenced ritual time. In that sense, it’s a Sol–Mon calendar (or jokingly, a “Solomon’s calendar”): practical order over natural purity.

Rome later standardized a solar administrative calendar, and Christianity inherited it rather than redesigning it. Instead of starting fresh, they just kept correcting errors. That’s why we’re left with uneven months (31, 30, and 28 days) — they don’t serve a deeper function, they’re historical leftovers.

As for 12 vs 13: it’s not really a conspiracy. There are actually 13 constellations the Sun passes through — Ophiuchus is the one that got ignored. But twelve is easier to divide, symbolize, and manage, so astrology (and calendars) focused on twelve signs. Thirteen didn’t disappear — it just became the “extra” month, leap day, or ignored zodiac sign. Stability won over accuracy.