In the rare case where the two prices are identical, the market effectively settles at a 50-50 outcome. It’s also worth mentioning that all figures will be rounded to the nearest cent, as per standard rounding rules.
Should SPX not trade at all during the regular session, the market will also settle at 50-50.
If any of the relevant days are shortened due to holidays, the S&P 500 Index will still use the official opening and closing prices from that session for final adjustments.
In instances where there’s no official closing price from the previous day—perhaps due to a trading halt, issues with the system, or other disruptions—the last valid traded price from the session will take precedence as the effective closing price.
The source for resolving this market is the Wall Street Journal, specifically looking at the opening and closing prices listed under “Historical Prices.”





