The S&P 500 is closing out 2025 with a rare and powerful show of momentum. As of Dec. 26, the benchmark index is up nearly 1% for the month, positioning itself for an eighth consecutive month of gains — a streak not seen since 2017 and achieved only a handful of times in the post-war era.The index of the 500 largest U.S. companies, tracked by the Vanguard S&P 500 ETF (NYSE:VOO) , still has three trading sessions left before the year ends. If it holds onto these gains, the rally that began last May will offi ...