spring day; day in spring
Refers to a day during the spring season, often with a poetic or gentle nuance.
spring sun; spring sunshine
Refers to the sun as experienced in spring, often implying warmth and gentleness.
The spring sun is shining through the window.
I like to take a walk in the park on a spring day.
Compound of 春 (spring) + の (possessive particle) + 日 (day/sun). The meaning depends on context.