For almost half of a century, Grass Valley residents and visitors have strolled the downtown streets to celebrate Cornish Christmas, and the 45th year of that tradition resumes Friday night. “It was the first big, Christmas-type celebration in the area,” said Julia Jordan, executive director of the Grass Valley Downtown Association, which coordinates Cornish Christmas. “It’s just tradition,” she said. Cornish Christmas takes place every Friday for five weeks starting the day after Thanksgiving and on into December.  During the weekly event, Mill and West Main streets are closed to traffic and filled with the sights and sounds of an …