I'm not a rocket scientist, but I would say likely that's when the launch window opened, or it was inside the window to be able to get the craft on the right trajectory to Mars. Launch windows take into account where the target's orbit is (in this case Mars) and where Earth is inside that window. In this case, the launch window was likely quite large due to Mars' orbit being quite a distance from Earth (and quite stable) which leaves quite a bit of time for correction burns in case the rocket is off course at all.