As you incorporate seasonal messaging into your marketing materials, keep in mind that seasons, as a general rule, aren’t capitalized. (“Take advantage of our fabulous spring specials.”)
If you’re incorporating the name of a season into the name of a special event or promotion, however, go ahead and capitalize. (“Our Spectacular Spring Sale starts this Saturday.”)
While we’re on the subject of caps, when your marketing is driven by holidays, religious events, and special occasions such as St. Patrick’s Day, Easter and Mother’s Day, caps are in order as these events, unlike seasons, are all proper names.