The fantasy basketball playoffs typically begin with roughly one month remaining in the NBA season. However, the playoff start date is arbitrary and can be determined by the league commissioner, who takes input from league members.
The reason the fantasy playoffs usually begin with several weeks remaining in the season is two-fold: 1. Time must be allowed for the playoffs to take place before the start of the NBA postseason; 2. Leagues typically prefer that the playoffs finish before the final week of the NBA season to prevent a situation in which teams are resting players or otherwise altering their regular rotation to prepare for the NBA postseason.