CME Group Equity Index futures "official" rollover date is today, December 12, 2019. The Emini S&P 500, Emini Nasdaq, Emini Russell and Emini Dow stock index futures contracts are rolling from the December 2019 contract to the March 2020 contracts. This is also true of the Micro Emini stock index futures contracts.
Remember futures contracts have an expiration date and eventually come off the board. The good news is that the same contract with a new expiration date takes on a position of primacy and is considered the new front month.
On the "official" roll date, volume starts to migrate from the old front month contract (in this case, the December 2019 contract) to the new front month contract (the March 2020 contract).
The stock index futures markets at the CME have quarterly dates, March, June, September and December (sometimes referred to as Christmas Spoos). Therefore, they rollover four times per year.
As today proceeds, you will see the volume migrate from the December 2019 to March 2020 contracts and that process will continue over the next couple trading days. Keep an eye on the volume and trade the more liquid month until the the new month takes control (i.e., has more volume). On the InfinityAT trading platform this is easy to identify by simply looking at the volume column in the quote board and comparing the December with the March contracts.
The traditional convention is to roll the expiring futures contract month (in the equities futures) eight calendar days before the contract expires. In this case, after doing the math, the December Stock Index Futures expire on December 20, 2019.
For most traders, it is important to trade the true front month contract month in order to take advantage of the greater liquidity.
Note: Most futures contracts (think Bonds, Currencies, Energies and Commodities) have different rollover conventions. If you are not sure or need additional information, please reach out to me at firstname.lastname@example.org.