With daily and weekly expirations, “OPEX” happens constantly — but the monthly expiration on the third Friday is a different animal, because it's where the largest, longest-accumulated open interest comes due.

Weekly OPEX: modest and routine

A standard weekly expiration clears a book that built over days to a couple of weeks. There's real charm-driven pinning into the Friday close, but the scale is manageable — the map reshuffles, and the following session opens without a dramatic reset.

Monthly OPEX: the big unwind

The monthly (and especially quarterly) expiration expires open interest that accumulated over months, including large index hedges and structured positions. Into it, pinning to major strikes and max pain can be strong; through it, a huge slice of the market's gamma disappears at once. The dealer hedges tied to all that expiring gamma unwind, and the positioning that had been anchoring price is suddenly gone.

A weekly OPEX reshuffles the deck. A monthly OPEX throws a chunk of it away — and the market that's left can move very differently.

What it means for the days around it

Into monthly OPEX, expect stronger pinning and range-compression near big strikes. After it, expect the character to change — the pinning gamma is gone, so the market often “unclenches” and moves more freely in the following days. Knowing whether a given Friday is a routine weekly or the monthly big one tells you which flow — and which post-expiration behavior — to expect.