Tessera

The planet on which the campaign takes place is called Tessera (adjective: Tessaran, inhabitants: Tesserans). Tessera is not Earth, but it's comparable to Earth in size, axial tilt, hours in a day, days in a year, climate, and proportion of landmass to oceans.

Tessera orbits a binary of two stars: a larger yellow dwarf called the Sovereign and a smaller red dwarf called the Tinker. The Sovereign and Tinker's rotation around their common barycenter is about 28 days.

Most of Tessera's solar insolation (i.e., warmth) comes from the Sovereign. However, during every fourth week the Tinker is partly or completely eclipsed by the Sovereign, resulting in significantly shorter days and colder temperatures--20 or more degrees Farenheit in some biomes. This effect is called the Chill.

Tessera has one moon called the Traveler with an orbit of 28-49 days. The Traveler has tidal effects on Tessera comparable to Luna's effects on Earth. Various conjunctions of the Sovereign, Tinker, and Traveler have significant effects on the physical and magical dimensions of Tessera.

(Here's a 1 minute video showing a planet orbiting a binary: https://www.youtube.com/watch?v=LTMKsQ63Bl8 )

(Here's what the binary might look like from space: http://www.ifa.hawaii.edu/info/press-releases/Kepler453b/PR_kepler453-sm.jpg )