How does perihelion occur?
Perihelion is the point in a planet's orbit when it is closest to its sun.
The shape of a planet's orbit around its sun is approximately an ellipse with the sun at one of the focii. Perihelion occurs when the planet is at its closest to its sun which happens every orbit.
Planetary orbits also undergo precession which means that the time between successive perihelia gets progressively longer each orbit.
Planetary orbits are not true ellipse due to the gravitational pull of other planets.
Earth is at perihelion around 3rd January. It can actually occur between 2nd and 4th January due to the gravitational effects of other planets.