From the introduction:
Imagine time as a river covered in ice, a rampaging current pushing everything from past to future without the thought of resistance, without any alternative. Now pretend that something had, somehow, poked holes through the cover - something strong enough to crack the ice of time.
In certain times and places, there are holes in the ice - small islands where things don't work like they should, where the world itself is not-quite-normal. For centuries, they've been considered curiosities and nuisances - periods where magic is dead, and even the power of the gods can't pierce. Various superstitions have surrounded them, and scholars have often debated their origin and meaning. Whole schools of math were created by wizards to predict where these islands will arrive next... so they won't be taken unaware. The gods themselves aren't talking.