On this day

./blog.sh on-this-day                    # today: every post from earlier years, and what the card shows
./blog.sh on-this-day --date 12-24       # another day of this year
./blog.sh on-this-day --date 2030-12-24  # ...or of another year, ages counted from then
./blog.sh on-this-day --json             # the same, as data

The same selection feeds a sidebar card, switched on by its name alone:

widgets:
  on_this_day:
    # heading: "On this day"    # optional -- the engine says it in every language

(or in ./style.sh, under the sidebar). The card reads the site's own archive, so there is no account to give and nothing to fetch. What it shows, and why each rule is there:

• Only the day itself. A post from an earlier year on today's date, in the site's time zone. A day with nothing from an earlier year hides the card, heading and all -- there is no "this week" fallback, which on a small archive showed one post seven days running. A post written on 29 February is remembered on the 28th in the years without the day.
• One row per year, five at most. Round anniversaries (5, 10, 15, 20 years...) always get a row. The rest are spread along the years with today counted as taken, so the older a year is, the stronger its claim -- "a year ago" appears when there is too little older to fill the card, not every day.
• A titled post stands for its year, the longest first; an untitled one only when the year has nothing titled that day. Titles decide inside a year and never between years: on an archive grown from several platforms they mark where a post came from, not how good it is.
• The day rotates. One window for every five posts the day has, three at most, dividing the day evenly by the clock (0-12-24, 0-8-16-24). Anniversaries stay; the other rows go first to years the day has not shown yet, and a year shown again brings its next post. Under the rows, the rest of the day folds away in a list.

Drafts, pages and unlisted posts never take part. The rule has no settings: it behaves the same on a hundred posts as on thirteen thousand, because what it depends on is how many years an archive spans and how many posts one year has on one day.

The card's file (on-this-day.json, in the root of every language's tree) is written by every build, and turned over after midnight by the sidebar cron below -- which is why a site with the card wants that cron even with no other widget. The cron chooses from a catalogue each build leaves beside the build cache (.on-this-day.<lang>.json), so it never needs to know what a translation is; a file from another day has no window for the page's clock and hides the card rather than show yesterday as today.