Day Zero is the day that the Arturians fired a Radiation Singularity Device in Earth’s sun.
When the device detonated it triggered a massive solar flare that burst out across the solar system.
When Earth was struck on the sunward-side the energy wave burned away all of the surface features, boiled the planet’s seas and oceans, and blasted away the atmosphere. Every living thing on the sunward-side died in an instant and without warning.
Atmospheric pressure dropped by over half and water vapor from the boiling seas enveloped the entire globe in less than ten minutes. The base temperature jumped by 60 degrees centigrade and killed almost everyone outside of shelter on the night side of the planet.
Radiation levels spiked to unhealthy levels. The Van Allen radiation belts were wrecked almost beyond recovery.
Objects in space were affected quite severely unless they were in the the shadow of a planetary body.
Space habitats, ships, and other facilities directly exposed to the flare were rendered lifeless in an instant. Radiation contamination made them useless from a salvage standpoint.
Outposts on other planets that were facing the sun during the event were devastated. Dark side facilities were mostly unscathed and served as calm ports in the storm during the subsequent rescue and evacuation efforts in-system.