The ninth edition of the MLA Handbook aims for stylistic consistency in prose and works-cited-list entries. Since seasons are styled lowercase in prose (e.g., My favorite seasons are spring and fall), seasons are styled lowercase in the Publication Date element of works-cited-list entries as well. Seasons will most likely appear in entries for journal articles, as shown in the following example:

Belton, John. “Painting by the Numbers: The Digital Intermediate.” Film Quarterly, vol. 61, no. 3, spring 2008, pp. 58-65.