Table schema:
CREATE TABLE table_name_7 (date VARCHAR, result VARCHAR, site VARCHAR, attendance VARCHAR)

Question: What is the Date at memorial stadium • minneapolis, mn with an Attendance of 20,000, and a Result of w52-0?
SQL: SELECT date FROM table_name_7 WHERE site = "memorial stadium • minneapolis, mn" AND attendance = "20,000" AND result = "w52-0"