Srimad-Bhagavatam: Canto 3 - Chapter 1 - Verse 20
Sanskrit:
सन्ध्यासन्ध्यांशयोरन्तर्य: काल: शतसंख्ययो: ।तमेवाहुर्युगं तज्ज्ञा यत्र धर्मो विधीयते ॥ २० ॥
ITRANS:
sandhyā-sandhyāṁśayor antaryaḥ kālaḥ śata-saṅkhyayoḥtam evāhur yugaṁ taj-jñāyatra dharmo vidhīyate
Translation:
The transitional periods before and after every millennium, which are a few hundred years as aforementioned, are known as yuga-sandhyās, or the conjunctions of two millenniums, according to the expert astronomers. In those periods all kinds of religious activities are performed.