Daily euro FX reference rates from the European Central Bank. ?base (default EUR), ?quote for a single rate, ?date=YYYY-MM-DD for history (last 90 ECB business days; older dates 404). Non-EUR bases are cross-rates derived via EUR (quote-per-EUR ÷ base-per-EUR). Weekend/holiday dates resolve to the last published ECB day — the date field says which. Published ~16:00 CET on ECB business days; informational rates, not for transaction pricing.