CREATE TABLE table_78821 ("Week" FLOAT, "Date" TEXT, "Opponent" TEXT, "Result" TEXT, "Kickoff Time" TEXT, "Attendance" TEXT)

What week was the opponent the San Diego Chargers?
SELECT AVG("Week") FROM table_78821 WHERE "Opponent" = 'san diego chargers'
SELECT AVG("Week") FROM "table_78821" WHERE "Opponent" = 'san diego chargers'