Hi logicgate,
S&P e-minis expire on Thursday prior to third Friday of the contract month. Usually traders roll over, to the next contract, about eight days before expiration date, but like one post above said - looking at volume is ussually the best way to roll. As soon as March 2019 will expire - that same day June contract of 2020 will start trading. Stock Index futures do not have many foreward expirations since almost all volume is traded in the current contract.
Good luck,
Simon