A light day is the distance light travels in one day in vacuum. It equals about 25.9 billion kilometers (specifically 25,902,068,370,000 meters, or 2.590206837×10^13 meters, which is 25.9×10^9 km). If you need a quick reference:

• In kilometers: approximately 25,902,068,370 km
• In miles: about 16,098,000,000 miles

If you want this expressed in astronomical units (AU) or parsecs, I can convert as well.