Refers to the evening or dusk of a spring day, often with a poetic or literary feel.
春の暮、桜の花びらが風に舞っていた。
On a spring evening, cherry blossom petals were dancing in the wind.
Refers to the late spring period, when the season is drawing to a close. Less common than the 'spring evening' sense.
春の暮になると、新緑が目立つようになる。
Toward the end of spring, fresh greenery becomes more noticeable.
Compound of 春 (spring) + の (possessive particle) + 暮 (dusk/end). The phrase can be interpreted either as 'spring's dusk' or 'spring's end', depending on context.