# 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".