S&P 500 (SPX) Opens Up or Down on May 21?

Platform comparison

Market context

Polymarket is pricing this contract at 0% for Up and 100% for Down, so the market is effectively saying the S&P 500 open on 21 May is already expected to print below the prior session’s close. On Polymarket, that view is expressed through USDC-backed conditional tokens on Polygon, so the spot price reflects where traders think the opening print will land relative to the last regular-session close, not where the index may end up later in the day.

For context, “open versus previous close” is a much tighter test than a normal intraday move, and it often turns on overnight futures, foreign market performance and the direction of the cash index at the opening bell. A 0%/100% split is extreme and usually implies either a very one-sided market or that trading liquidity has thinned enough for the quote to harden. The NYSE had the S&P 500 at 7,403.05, down 0.07%, in its morning market snapshot today, which is consistent with a modestly softer tone rather than a large gap in either direction.

The main things to watch before the 8 p.m. UTC settlement window closes are any late-day move in index futures, Treasury yields and headlines from Washington or the Federal Reserve that could shift the opening auction. The market’s reference is the official S&P 500 opening price, so the key dependency is the exchange-determined open, not the first trade after the bell. If the pre-market tone changes sharply after scheduled data, earnings or policy comments, the contract can still move quickly on Polygon as traders adjust their USDC positions in the conditional-token market.

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Methodology

This page reviews S&P 500 (SPX) Opens Up or Down on May 21? across five venues. We show live odds for Polymarket-based markets (sourced from the Polygon order book); for other venues we list platform attributes, since the comparable contracts are not exposed via a public API on every venue. Every CTA points at PolyGram — the application we operate, where you trade directly against the Polymarket order book at 0% fees.

Resolution & payout

Polymarket-based markets settle through the UMA Optimistic Oracle on Polygon. A proposer submits the outcome, a two-hour challenge window opens, and unchallenged proposals finalise the resolution. Payouts settle automatically in USDC the moment the result is final — no bookmaker, no delay.

Kalshi-based markets settle in USD via the CFTC-regulated clearinghouse. Betfair Exchange settles in GBP/EUR net of commission. Manifold is play-money and does not pay out real funds.

FAQ

Where can I trade this market with the lowest fees?

On PolyGram, which mirrors the Polymarket order book at 0% fees. Kalshi charges up to 7% per trade; Betfair Exchange takes 2-5% commission on net winnings.

How does resolution work?

Through the UMA Optimistic Oracle on Polygon: a proposer submits the outcome, a two-hour challenge window opens, and USDC payouts settle automatically once the result is final.

What's the difference between YES and NO shares?

A YES share pays $1.00 if the event happens, $0 otherwise. A NO share pays $1.00 if the event doesn't happen. The market price between 0¢ and 100¢ is the implied probability.

What does it cost to trade on PolyGram?

Zero. PolyGram routes every order to the live Polymarket order book; the only cost is the Polygon network fee, typically under $0.01 per transaction.

How reliable are the quoted odds?

The YES/NO percentages are the live mid-prices of the Polymarket order book. On deep markets they move every few seconds; on thinner ones you'll see short plateaus.