SB 9.14.43

sthālīṁ nyasya vane gatvā
gṛhān ādhyāyato niśi
tretāyāṁ sampravṛttāyāṁ
manasi trayy avartata

sthālīm—the woman Agnisthālī; nyasya—immediately giving up; vane—in the forest; gatvā—on returning; gṛhān—at home; ādhyāyataḥ—began to meditate; niśi—the whole night; tretāyām—when the Tretā millennium; sampravṛttāyām—was just on the point of beginning; manasi—in his mind; trayī—the principles of the three Vedas; avartata—became revealed.

King Purūravā then left Agnisthālī in the forest and returned home, where he meditated all night upon Urvaśī. In the course of his meditation, the Tretā millennium began, and therefore the principles of the three Vedas, including the process of performing yajña to fulfill fruitive activities, appeared within his heart.

It is said, tretāyāṁ yajato makhaiḥ: in Tretā-yuga, if one performed yajñas, he would get the results of those yajñas. By performing viṣṇu-yajña specifically, one could even achieve the lotus feet of the Supreme Personality of Godhead. Of course, yajña is intended to please the Supreme Personality of Godhead. While Purūravā was meditating upon Urvaśī, the Tretā-yuga began, and therefore the Vedic yajñas were revealed in his heart. But Purūravā was a materialistic man, especially interested in enjoying the senses. Yajñas for enjoyment of the senses are called karma-kāṇḍīya-yajñas. Therefore, he decided to perform karma-kāṇḍīya-yajñas to fulfill his lusty desires. In other words, karma-kāṇḍīya-yajñas are meant for sensuous persons, whereas yajña should actually be performed to please the Supreme Personality of Godhead. To please the Supreme Personality of Godhead in Kali-yuga, the saṅkīrtana-yajña is recommended. Yajñaiḥ saṅkīrtana-prāyair yajanti hi sumedhasaḥ [SB 11.5.32]. Only those who are very intelligent take to saṅkīrtana-yajña to fulfill all their desires, material and spiritual, whereas those who are lusty for sense enjoyment perform karma-kāṇḍīya-yajñas.