Final answer:
The mood of the stanza from Longfellow's "An April Day" is best described as serene, depicting a peaceful and calm atmosphere with the warmth of the sun and the first flowers of spring.
Step-by-step explanation:
The word that best describes the mood of the stanza from Longfellow's "An April Day" is serene. The stanza speaks about the sweetness of visiting the still woods and the first flowers of the plain, which suggests a peaceful and calm atmosphere. The warm sun and the reappearance of seed-time and harvest convey a sense of renewal and growth, which is typical of spring and creates a serene mood.