Fashion standards may decree that one never wear white after Labor Day, but more than 5,000 New Yorkers ignored that memo yesterday as they descended upon Robert F. Wagner Jr. Park for the sixth annual Diner en Blanc. The guests — who were informed of the event’s secret location just one hour before it was scheduled to begin — arrived with fanciful costumes. Decked out in head-to-toe white, often with fanciful accessories, they also brought picnic tables and elaborate illuminated place settings. Sitting down to their meals as the sun set, the champagne flowed freely and dancing into the night quickly followed.