December is historically one of the best months of the year for the SPDR S&P 500 ETF Trust. However, most of those historical gains have come in the second half of the month in a seasonal trade known as the Santa Claus Rally. The official period of the Santa Claus Rally is the final five trading days of December and the first two trading days of January. This year, the Santa Claus Rally period begins on Thursday, Dec. 24. The S&P 500 has traded higher during this seven-day stretch in 77.9 percent of years. In addition, the S&P 500 has averaged a 1.33 percent gain during this period, making it the second-best seven-day stretch of the year for stocks, according to LPL Financial. “Whether optimism over a coming new year, holiday spending, traders on vacation, institutions squaring up their books before the holidays—or the holiday spirit—the bottom line is that bulls …