user: Question: What was the main date of the round with 20 fixtures?
Columns: ['Round', 'Main date', 'Number of fixtures', 'Clubs', 'New entries this round', 'Prize money']
Types: ['text', 'text', 'real', 'text', 'text', 'text']
Sample row: ['Extra Preliminary Round', '16 August 2008', 203.0, '761 → 558', '406: 356th–761st', '£750']
SQL:

assistant: SELECT "Main date" FROM "Table" WHERE "Number of fixtures" = 20.0;