It's a terrible proposition to buy long calendars if vol has peaked and dropping. It's long volatility and short gamma. Cutten qualified short volatility/long delta which are diametric-conditions for a long index calendar. Even a strike-pin [gamma-win] isn't likely to compensate for the loss to vol [delta loss, dV/dVol].
In case you were confused by atticus' post, long atm cals benefit from rising vol whereas short cals benefit from falling vols. The closer to expiry you are with the short leg of the long (debit) cal the more expensive the cal debit will be and thus the higher your risk. db
Thanks for the explanation. Will the strike make a difference in selecting the strategy? Or the expected degree of rebounce?