Del Mar Thoroughbred Club, July 17th. Opening Day at Del Mar is the day the racing season begins and the track reminds everyone why they've been waiting since last September. The hats come out. The paddock fills with horses that have been training all spring for exactly this track, this turf, this stretch. The summer meet opens and Del Mar is itself again. Opening Day is a cultural event before it is a racing event. The fashion, the tradition, the specific San Diego energy of a day at the track when the sun is full and the bay is visible beyond the finish line — this is the Friday afternoon …