Mandate review: S&P 500 close 2026-10-20 vs 2026-10-16

Verdict: ACCEPT

The question can be judged fairly after its deadline by independent researchers who use different sources and methods.

Facts (checked against the calendar, not against market data)

• 2026-10-16 is a Friday and 2026-10-20 is a Tuesday, as the question says.
• Both are ordinary weekdays. Neither is a standard US market holiday in the NYSE calendar, as far as I know. A researcher should confirm this against the NYSE holiday calendar.
• US daylight time runs until 2026-11-01, so the cash close of 16:00 ET is 20:00Z. The deadline of 21:30Z falls 90 minutes after the close, once the official closing level is normally published.
• I did not retrieve or use any index levels. The mandate does not need them, and the outcome is not known at the time of writing.

Why it is fair to judge

• The outcome is binary and comes from one numeric comparison. YES means close(10-20) > close(10-16). Equal or lower is NO.
• The rule is objective and strict, so ties go to NO. The only INVALID condition is cancellation of either regular session.
• The primary source is named (S&P Dow Jones Indices). The index close is public, and any researcher can check it from many aggregators.
• Researchers need no private data. They can reach the same answer by different routes, and their reasoning about direction can differ without affecting the resolution.

Inferences and uncertainty

• The §resolution line says "subjective expert judgment", but the rule is mechanical. I read this as: researchers may choose their own sources and methods, but the result still follows the §rule comparison. Expert judgment should only decide which evidence to trust when sources conflict.
• The mandate does not say which series counts. I read "S&P 500 index" as the price index (SPX), not total return or an ETF. This is the natural reading, but it is an inference.
• Closing levels can occasionally be corrected after publication. The deadline is the same evening, so a researcher should record the time they read the value.
• Aggregators sometimes show a stale or after-hours value. The official S&P DJI close wins in a conflict. This is already implied by the source line.

Unanswered questions

• Whether either date is an unscheduled closure (for example, an emergency halt of the whole day). This can only be known on the day, and the INVALID clause covers it.
• A mid-session halt or early close is not cancellation, so the question would still resolve normally.

Optional wording tightening (not required for acceptance)

1. State "S&P 500 price index (SPX), not total return".
2. State "if sources disagree, the S&P DJI official close prevails".
3. Reword §resolution to: "researchers choose their own sources and methods; the result must follow the §rule comparison".