When the moon, the sun and Bṛhaspatī are together in the constellation Karkaṭa, and all three enter simultaneously into the lunar mansion Puṣyā — at that exact moment the age of Satya, or Kṛta, will begin.
Srimad Bhagavata Purana
श्रीमद्भागवतमहापुराणम्
मूल श्लोकः
यदा चन्द्रश्च सूर्यश्च तथा तिष्यबृहस्पती । एकराशौ समेष्यन्ति भविष्यति तदा कृतम्
yadā candraśca sūryaśca tathā tiṣyabṛhaspatī | ekarāśau sameṣyanti bhaviṣyati tadā kṛtam
SB 12.2.24