Dec 31 (Reuters) - Canada’s main stock index opened lower on the last trading day of the year on Thursday, dragged by weakness in energy stocks, while investors focused on fading prospects for bigger U.S. stimulus checks. * At 09:30 a.m. ET (14:30 GMT), the Toronto Stock Exchange’s S&P/TSX composite index was down 21.89 points, or 0.12%, at 17,523.92.