CREATE TABLE table_204_513 (id NUMBER, "date" CLOB, "time" CLOB, "opponent#" CLOB, "site" CLOB, "tv" CLOB, "result" CLOB, "attendance" NUMBER)

when was the first time the attendance was below 20,000 ?
SELECT "date" FROM table_204_513 WHERE "attendance" < 20000 ORDER BY "date" LIMIT 1
SELECT "date" FROM "table_204_513" WHERE "attendance" < 20000 ORDER BY "date" FETCH FIRST 1 ROWS ONLY