How Rokuyo (Taian, Butsumetsu) Is Calculated from the Lunar Calendar Japan's rokuyo (Taian, Butsumetsu…) aren't decided by daily divination — they're computed from the lunar calendar. The formula, plus a real 2026 distribution we counted ourselves. Why the Solar Terms Shift Every Year — and the Setsubun That Moved The 24 solar terms track the Sun's position, so their dates drift. A ~60-year distribution of risshun we computed shows why 2021 and 2025 shifted to Feb 3. How Japan's Public Holidays Are Decided — Substitute Holidays and the Equinox Japanese holidays follow the Holiday Law, not a fixed table. We explain substitute and 'citizens' holidays with a computed list of the years they actually occur. The Pitfalls of Japanese Era Conversion — the Showa/Heisei/Reiwa Boundaries Japanese eras switch on a specific day, not a year. A table of era-change dates and boundary conversions clears up the mistakes forms often make. The Kyureki 2033 Problem — When the Lunar Calendar's Months Become Undefined Under the Tenpo rules, the lunar month names for late 2033–2034 aren't uniquely defined. We show the solstice-priority method Benri.dev uses and how methods diverge. A '6-Tatami Room' Isn't One Size — Kyoma, Chukyoma, Edoma, Danchima Tatami come in four regional standards, so a '6-tatami' room can differ by over 10%. A computed m² comparison from real tatami dimensions clears the confusion. Yakudoshi Runs on Kazoedoshi — Counted vs. Actual Age, with Tables Yakudoshi 'unlucky years' are counted the old way. How kazoedoshi differs from actual age, plus computed tables of the birth years currently in their main year. 10 Common Regex Pitfalls — Greedy Matching, Newlines, Escaping Greedy quantifiers, dot vs. newline, escaping inside classes — ten regex traps, each shown as input / intended pattern / actual match / fix. A Field Guide to Invisible Characters — Zero-Width, BOM, and NBSP Searches that won't match, diffs that lie, paste that breaks — invisible characters are often the cause. A table of code points, sources, and risks. Password Strength Is Mostly Length — An Intro to Entropy For passwords, length beats complexity. We compute entropy = log2(charset^length) and tabulate brute-force times by charset and length for practical guidance.