Weekly Options Premium Decay
Weekly options premium decay described the accelerated rate at which time value eroded in short-dated Nifty and Bank Nifty weekly options during the final two to three trading days of the expiry cycle, a characteristic that premium sellers historically sought to exploit through positions established earlier in the week.
The theta decay formula for options showed that time value erosion was not linear but accelerated as expiration approached. For a standard at-the-money option, the daily theta was proportional to the square root of time remaining rather than time itself, meaning that options lost a disproportionately large fraction of their remaining value in the final days. For weekly options on Nifty — which had a five-day or shorter life — this dynamic was compressed into an extremely short window.
Empirically, Indian F&O participants observed that an at-the-money Nifty weekly straddle sold on Monday would retain a significant portion of its premium through Tuesday. By Wednesday, decay began to accelerate visibly. On the final day (Thursday for most of Nifty's weekly history), the remaining time value could collapse by 50-70% within the first hour of trading if the index remained near the sold strike. This dramatic intraday decay in the last session became a defining feature of weekly options trading.
Premium sellers attempted to capture this final-day decay by establishing short straddle or short strangle positions on Monday or Tuesday, targeting the natural time erosion through the week. The risk to this approach was that gamma risk was simultaneously highest in the final days. A gap-up or gap-down opening on the expiry day could instantly wipe out multiple days of accumulated theta gain if the index opened well beyond the short strikes.
The introduction of multiple weekly expiries across different indices — with SEBI rationalising to one weekly expiry per exchange — meant that from October 2023, Nifty weekly options concentrated all weekly decay activity into the single NSE-designated weekly expiry cycle. This concentration increased the significance of the Nifty weekly theta decay window and removed the alternative Bank Nifty Thursday decay trade that had previously attracted a separate segment of premium sellers.
Options buyers on the final expiry day, conversely, were aware that purchasing at-the-money options with a few hours to expiry was a high-risk, high-reward proposition. The premium might be small in absolute terms, but delta and gamma were both near their maximum values, meaning even a modest intraday move could multiply the option's value rapidly.