Srimad Bhagavatam Canto 03, Chapter 11, Text 20

SB 3.11.20

sandhya-sandhyamsayor antar
 yah kalah sata-sankhyayoh
tam evahur yugam taj-jña
 yatra dharmo vidhiyate
 
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada: 
 
The transitional periods before and after every millennium, which are a few hundred years as aforementioned, are known as yuga-sandhyas, or the conjunctions of two millenniums, according to the expert astronomers. In those periods all kinds of religious activities are performed.
Srimad Bhagavatam Canto 03, Chapter 11, Text 19
Srimad Bhagavatam Canto 03, Chapter 11, Text 21