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

Platform comparison

Market context

The contract is effectively priced as a full certainty that the S&P 500 will finish higher on 21 May, with the market showing 100% YES. On Polymarket, that means traders have already pushed the USDC-denominated, Polygon-settled conditional tokens to the top of the range: anyone buying in now is paying on the assumption that the official close from the S&P Dow Jones index will be above Wednesday’s close, and the only question left is whether there is any meaningful room for downside surprise before the settlement window shuts at 20:00 UTC.

That near-unanimous pricing is unusual for a one-day directional market, but it does fit a backdrop in which U.S. equities have been making fresh highs. The S&P 500 has traded above 7,200 this week, and the Nasdaq and S&P 500 both posted strong gains in April after investors reacted to firmer economic data, easing Middle East tensions and solid first-quarter earnings. In that sort of tape, a daily “up or down” contract often gets crowded to one side because traders anchor to the prevailing trend rather than the next close, even though single-session reversals are common around large index levels.

For a trader watching this specific settlement, the main inputs are the morning futures tone, any late-day move in Treasury yields or oil, and the size of the cash close relative to the previous session. 247WallSt reported earlier today that stocks had slipped as oil jumped and the Nasdaq fell 0.55%, which is a reminder that a strong open does not guarantee an up close. Any last-minute macro headline, major earnings release, or move in mega-cap names before the 20:00 UTC cutoff can still matter, because the market resolves only on the official index close, not on intraday highs or the Polymarket price itself.

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Methodology

This page reviews S&P 500 (SPX) 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

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

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.

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.

Do I need to KYC for this market?

Not under $1,500 of lifetime trading volume. Above that threshold, PolyGram triggers a quick verification flow that finishes in 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.