The word "estival" refers to something that is related to summer, such as warm weather or a summer season. The antonyms of "estival" are words that are related to winter or the winter season. These may include words such as frigid, icy, chilly, frosty, arctic, polar, hibernal, wintry, and nippy. These words represent the opposite of summer, describing cold weather and dark nights. Other antonyms of "estival" may include autumnal, spring-like, and vernal, which all relate to the seasons of autumn and spring. The use of antonyms can help to convey a sense of contrast and provide a more vivid description of the topic.