"Following the UK decision to relinquish the Council presidency in the second half of 2017, the Council decided to bring forward by six months the order of presidencies, starting from 1 July 2017. It also
“Following the UK decision to relinquish the Council presidency in the second half of 2017, the Council decided to bring forward by six months the order of presidencies, starting from 1 July 2017.
It also decided to add Croatia, which was not yet a member state at the time of the original decision, for the period January-June 2020.”
See: Council rotating presidencies: decision on revised order (link)