A poetic or literary expression for a spring evening, often evoking a calm, beautiful, or fleeting atmosphere.
春の宵、桜の下をそぞろ歩く。
On a spring evening, I stroll under the cherry blossoms.
Compound of 春 (spring) + の (possessive particle) + 宵 (evening, early night). 宵 is a somewhat literary term for evening, giving the phrase a refined tone.