The story
This is the night of the rāsa: Kṛṣṇa played the flute at Vṛndāvana and every gopī who came found him beside her, because he made himself as many as there were.
In Bengal and Odisha the night belongs to Lakṣmī, who walks the earth asking ko jāgarti — who is awake? — and enters the house of whoever answers. Kojāgarī is that question.
The moon is held to drop something medicinal on what is left under it, which is why the kheer is left out and eaten cold in the morning rather than warm at night.