user: Question: What is the date on which a race was run at Halle-Saale-Schleife circuit?
Columns: ['Race Name', 'Circuit', 'Date', 'Winning driver', 'Constructor', 'Report']
Types: ['text', 'text', 'text', 'text', 'text', 'text']
Sample row: ['I Rostock Osthafenkurs', 'Rostock', '20 April', 'Paul Greifzu', 'BMW - Eigenbau', 'Report']
SQL:

assistant: SELECT "Date" FROM "Table" WHERE "Circuit" = 'Halle-Saale-Schleife';