These words refer to a festival or feast, but festive has an added meaning of "joyous" or "merry." That is, a festal occasion may be festive, but not all festive occasions are connected with a feast or festival: "Decorations for the dance were imaginative and festive." "The reunion of our class was a festive occasion." Many festal rites are connected with such occasions as Christmas, Easter, and Passover: "Following church services, the congregation will sit down to a festal meal."
Dictionary of problem words and expressions. Harry Shaw. 1975.