CREATE TABLE table_29637 ("Season" TEXT, "Episodes" FLOAT, "Timeslot  (  EST  ) " TEXT, "Season premiere" TEXT, "Season finale" TEXT, "TV season" TEXT, "Rank" TEXT, "Viewers  ( in millions ) " TEXT)

When was the season finale that ranked 20th?
SELECT "Season finale" FROM table_29637 WHERE "Rank" = '20th'
SELECT "Season finale" FROM "table_29637" WHERE "Rank" = '20th'