On this day (lib/on_this_day.rb)

One module decides which posts from earlier years stand on a date and which of them the card shows; the command (scripts/on_this_day.rb), the build and the cron all call it, so what the author sees in the terminal is what a reader gets. It is data in, data out: entries (day, year, time, titled, word count, plus whatever the caller carries) and a date go in, windows of rows come out. It never reads the config and never decides a language.

The split between build and cron is the part worth knowing. What a post is called and where it lives depends on the language -- a translation's own title and address, a fallback's, the site's -- and only the build knows that. The day, on the other hand, changes at midnight, when only the cron runs. So every build run writes a catalogue for its language beside the build cache (.on-this-day.<lang>.json: every listed post with its selection facts, its title and its address in that language, and the tree it belongs to), plus today's file; the cron re-chooses from the catalogues and rewrites a file only when its content changed. A build with the card off removes its catalogue, and the run that owns the site root removes catalogues of languages the site stopped publishing -- otherwise the cron would keep writing a tree nobody builds.

The page's file carries every window with its start and end in UTC, and sidebar.js shows the one the clock is in. That is also the staleness check: yesterday's file has no window for now, so a stopped cron hides the card instead of showing the wrong day.