CREATE TABLE table_6226 ("Week" FLOAT, "Date" TEXT, "Opponent" TEXT, "Result" TEXT, "Game site" TEXT, "Attendance" FLOAT)

Opponent of denver broncos happened on what date?
SELECT "Date" FROM table_6226 WHERE "Opponent" = 'denver broncos'
SELECT "Date" FROM "table_6226" WHERE "Opponent" = 'denver broncos'