What is a perihelion?
A planet is at perihelion when it is at the closest point in its orbit to its sun.
Planets orbit around their sun in approximately elliptical orbits. The sun is at one of the focii of the ellipse. The point in the orbit when the planet is at its closest to its sun is called perihelion. The point where it is furthest from the sun is called aphelion.
Most planetary orbits are not true ellipses due to the gravitational effects of other planets. The point of perihelion also precesses which means that the point of perihelion gets a bit later after each orbit.
Earth reaches perihelion around the 3rd of January each year, though the actual date and time varies dependent on the effects of other planets. Due to precession the Earth's perihelion gets a day later every 50-60 years.