On This Day uses the Wikipedia REST feed/onthisday endpoint. Requests go out by event type and by month/day, preferring the Wikipedia data source that matches the current page language; only when that language's OnThisDay feed is unavailable or the request fails does it fall back to English. Results are cached for about 24 hours per language, type and month/day.

1. 01Choose a date and an event type.
2. 02The page requests the historical events for that month and day and shows the year, a summary and related pages.
3. 03When the data source for the current page language is unavailable, English data is tried automatically.

• The data depends on what Wikipedia OnThisDay holds and on the endpoint being available; it is not a complete historical database.
• Different language editions of Wikipedia differ in how many events they list, which ones they choose and how they word them; the page says so explicitly when it falls back to English.
• The year input drives the page interaction and must not be read as “query that year”: the endpoint's dimensions are month and day.
• The 24-hour local cache can briefly keep an older result.
• Wikimedia has marked the Wikifeeds API as being deprecated progressively from July 2026, so the current endpoint should be treated as a transitional dependency that ought to migrate once a replacement route is settled.

Wikipedia receives the query type, the month and the day, never the year you selected on the page; results may be cached in the current browser's localStorage.