Moreover, the fifth day of May, as shown on Granted Calendar, is in fact letter s, the initial of summer. As you may know, luna calendar have 24 solar terms spread over any given year. By evenly subdividing ecliptic into 24 parts, we get 24 points. Each point occupies a certain date. Each point tells an important seasonal change, like spring begins, more rain than snow, hibernating insects awaken etc.. Unsurprisingly, 5th May is summer begins.