user: Question: What is the circuit held on 15 April?
Columns: ['Race Name', 'Circuit', 'Date', 'Winning driver', 'Constructor', 'Report']
Types: ['text', 'text', 'text', 'text', 'text', 'text']
Sample row: ['II Race of Champions', 'Brands Hatch', '12 March', 'Dan Gurney', 'Eagle - Weslake', 'Report']
SQL:

assistant: SELECT "Circuit" FROM "Table" WHERE "Date" = '15 April';