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

Platform comparison

Market context

Polymarket is pricing this contract as a one-way move: the crowd-implied probability is 0% for Up, so holders of the down side are effectively assigning no weight to the S&P 500 opening above Wednesday’s close. On Polymarket, the position is settled in USDC on Polygon through conditional tokens, so the only thing that matters at resolution is the official SPX open versus the prior trading day’s official close, not where futures trade beforehand or where cash equities later finish.

For context, SPX open-vs-prior-close markets often hinge on overnight positioning rather than a broad reassessment of fundamentals. A flat or modestly negative cash session can still be followed by an up open if futures firm into the auction, while a large gap tends to come after a heavy macro or rates impulse. The current 0% YES price suggests the market sees the open as fully committed to downside, which is unusual in a daily SPX event and leaves little room for a surprise pre-open bounce to matter to traders.

The main catalysts to watch are the overnight S&P futures move, Treasury yields, and any late data or policy headlines before the opening auction. NYSE’s morning market summary showed the index slightly lower earlier today, while S&P 500 futures will be the closest live read on whether cash can recover by the open. Traders also need to watch the US economic calendar and any Federal Reserve commentary, because even a small shift in rate expectations can change the opening print, especially when the settlement window ends at 20:00 UTC and the contract resolves purely on the official opening level.

Sources: 1 · 2 · 3 · 4

Methodology

This page is a comparison snapshot: one live quote (Polymarket), four reference venues with their key attributes, and a single execution path — every trade button routes to PolyGram, which mirrors the Polymarket order book directly.

Resolution & payout

Settlement runs on-chain. Polymarket's contract logic separates YES and NO shares as conditional tokens; at resolution the winning share lifts to $1.00 and the losing one to $0. The outcome input comes from the UMA Optimistic Oracle, which secures against bad resolution with a bond + dispute window.

Once finalised, the smart contract pays USDC to the holders' wallets within minutes — no withdrawal fees beyond Polygon network gas. Kalshi settles in USD via CFTC clearance, Betfair in account currency net of commission, Manifold in play-money mana with no cash-out.

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.

Is this market available outside the US?

PolyGram is available in most jurisdictions where Polymarket isn't directly accessible. Polymarket itself is geo-blocked in the US/UK/EU. Always check local regulations.

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.

How fast are USDC deposits?

Polygon credits deposits after 12 confirmations — usually under 30 seconds. Withdrawals follow the same path and land back in your wallet within minutes.

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.