Financial futures (such as emini S&P, emini Dow) and commodity futures are traded in contracts, and the contracts expire every three months on financial futures. The current front month for ES (emini S&P) is September, and on the 17th it expires. Rollover day is one week and one day before this, and this is the date when all the main volume being traded switches from the September contract to the December contract.
This means that you will see very little volume on the September contract tomorrow as everyone (or virtually everyone) is trading December. It means that you need to change your software codes for charts, and to change your default on your trading platform to ensure you are trading December contracts, not September.
As the September contract expires on Friday 17th, it is called Witching. There are four types of contract which expire on that day, and so it is Quadruple Witching.
During Witching the market can be very volatile, particularly before 12:00 noon ET time, so you need to fasten your seatbelt tightly. Best stay out of the market if you don't know what to expect.
