In Tucson, shorts weather never completely ends; there's at least one day even in December and January when one can wear shorts at least part of the day. And 100-degree weather sometimes shows up in March, and it can stay hot well into October. But Memorial Day is special. By then, the snowbirds have all gone back home for the summer, the university has held final exams and commencement, and the streets belong to the locals again. That's the beginning of summer.