user: Question: When was an average speed of 81.388 mph recorded?
Columns: ['Year', 'Date', 'Driver', 'Team', 'Manufacturer', 'Laps', 'Miles (km)', 'Race Time', 'Average Speed (mph)', 'Report']
Types: ['text', 'text', 'text', 'text', 'text', 'text', 'text', 'text', 'text', 'text']
Sample row: ['1961', 'July 30', 'Jack Smith * Johnny Allen *', 'Jack Smith', 'Pontiac', '500', '250 (402.336)', '3:39:23', '68.373', 'Report']
SQL:

assistant: SELECT "Date" FROM "Table" WHERE "Average Speed (mph)" = '81.388';