te upetya maha-ratre
urvasya uranau jahrur
nyastau rajani jayaya
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
Thus the Gandharvas came to earth, and at midnight, when everything was dark, they appeared in the house of Pururava and stole the two lambs entrusted to the King by his wife, Urvasi.
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
“The dead of night” refers to midnight. The maha-nisa is described in this smrti-mantra: maha-nisa dve ghatike ratrer madhyama-yamayoh, “Twelve o’clock midnight is called the dead of night.”