The parameters of fantasy football playoffs aren't standard from league to league.
In many instances, they run from Weeks 15-17, with six teams getting berths and the top two seeds receiving byes during Week 15. Then Week 16 hosts two semifinal matchups, while the championship game occurs in Week 17. (Leagues often choose not to include Week 18 in the fantasy playoffs, as many NFL teams choose to rest their star players, making many fantasy football stars unstartable.)
In leagues in which just four teams make the postseason, a common setup uses the combined scores from Weeks 15 and 16 to determine semifinal winners, before taking the combined Week 17-18 score to figure out the champion.