The Earth reaches its greatest orbital speed when it is closest to the Sun, at the point in its orbit called perihelion.

This might seem counterintuitive, but it's a result of Kepler's Second Law of Planetary Motion. This law states that a planet sweeps out equal areas in equal times. Since the Earth's orbit is slightly elliptical, it moves faster when it's closer to the Sun (covering a larger area) and slower when it's further away.

Therefore, the Earth's orbital speed is greatest around early January, when it's at perihelion.