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

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" NULLS FIRST LIMIT 1