Euphemistic, literary expression for sexual activity, literally 'night's work' or 'nightly activities'. Used to refer to sex indirectly, often in a poetic or discreet manner.
The couple cherishes their nightly intimacy.
The novel contains descriptions of sexual encounters.
Compound of 夜 (yoru, 'night') + の (no, possessive particle) + 営み (itonami, 'work, activity, conduct'). The phrase literally means 'night's activities' and is used euphemistically to refer to sexual intercourse.