उर्वश्या उरणौ जह्रुर्न्यस्तौ राजनि जायया ॥ २७ ॥
urvaśyā uraṇau jahrur
nyastau rājani jāyayā
Thus the Gandharvas came to earth, and at midnight, when everything was dark, they appeared in the house of Purūravā and stole the two lambs entrusted to the King by his wife, Urvaśī.
“The dead of night” refers to midnight. The mahā-niśā is described in this smṛti-mantra: mahā-niśā dve ghaṭike rātrer madhyama-yāmayoḥ, “Twelve o’clock midnight is called the dead of night.”