Large institutions — pensions, target-date funds, balanced portfolios — rebalance to fixed asset-allocation weights on a schedule, and a lot of that happens in the final day or two of the month. These are price-insensitive flows: they trade to hit a target, not because they have a view. That size interacts with the dealer map.

Why month-end matters

If equities rallied during the month, rebalancers may need to sell stocks to trim back to target (and vice versa after a down month). The direction depends on the month's performance, but the timing is predictable: concentrated flow into the last session or two, often into the close. It's one of the more reliable calendar effects because it's driven by mandates, not sentiment.

The gamma backdrop decides the outcome

The same rebalancing flow lands very differently depending on the regime it hits. In positive gamma, dealer hedging absorbs the flow — the rebalance gets dampened into a grind. In negative gamma, the same flow gets amplified — a rebalancing sell can feed a sharper drop. The flow is the fuel; the gamma regime is the accelerant or the brake.

Month-end flow is scheduled and price-insensitive. Whether it grinds or gaps is up to the gamma regime it walks into.

How to use it

Know the date, know the month's trend (to guess the rebalance direction), and — most important — read the regime to know how the flow will land. A month-end sell into positive gamma is a fade-the-dip setup; the same sell into negative gamma is a respect-the-trend one. Same calendar event, opposite trade, decided by the map.