Dr. Sergei Koussevitzky, who is returning to America after opening the Sibelius Festival in Helsingfors, Finland, will begin his twelfth season with the Boston Symphony Orchestra as he opens the Friday afternoon Saturday evening series on October 11 and 12. The Monday evening series of six concerts will open on October 28, and the Tuesday afternoon series on November 5. The concerts under the auspices of Harvard University will be played in Sanders Theatre as usual, commencing on October 17.