Although we don't believe in timing the market or panicking over market movements, we do like to keep an eye on big changes -- just in case they're material to our investing thesis.

This is shaping up to be an ominous start to third-quarter earnings season after the bell, with the S&P 500 (INDEX: ^GSPC) delivering one of its worst slides over the past month.

Today was absolutely marred with bad news from nearly all angles. The ongoing government shutdown stymied what little economic data we would have expected to see today, allowing investors to focus entirely on the fact that neither party is making any headway. Comments from both parties appear to point to no easy resolution and the likelihood that we'll still be talking about a shutdown at this time tomorrow. Any protracted shutdown could negatively affect U.S. GDP and practically every component within the S&P 500.

In addition, it was an absolute nightmare of a day for social-media stocks. Call it a tech-kabob if you will, but the Global X Social Media Index ETF (NASDAQ: SOCL) dropped more than 4% as some of its largest holdings, including Facebook, LinkedIn, and Pandora Media, were creamed. This marked the worst fall for social-media stocks and the Global X Social Media Index ETF in approximately 17 months and serves as a reminder to investors that valuations, no matter how quick the growth rate, are still important -- especially so when the prospect of a debt-default is on the horizon.

By day's end, the S&P 500 simply couldn't hold up under the weight of this negative news and ceded 20.67 points (-1.23%) to close at 1,655.45, its 11th loss in the past 14 sessions.

As you might imagine, finding strong stocks today that bucked the trend wasn't easy, but energy-drink maker Monster Beverage (NASDAQ: MNST) handily topped that list with a gain of 3.7%. The catalyst for the move appears to be positive comments made by BMO Capital Markets analyst Asit Sharma, who notes that IRI data would indicate Monster's growth remains strong and rumblings of a slowdown are unwarranted. While I do share some of Sharma's enthusiasm, I'm not certain that energy-drink makers like Monster have completely escaped the ire of regulators just yet with regard to the health effects of their drinks. As such, I'm still keeping a safe distance between my money and Monster Beverage shares.

Shares of drug distributor McKesson (NYSE: MCK) popped 3.2% higher after word got out that it's in possible talks to acquire European rival Celesio for about $5.1 billion. A deal would certainly make sense from the standpoint of both companies, since McKesson is looking to expand its pricing power and geographical reach to Europe, while Celesio has struggled with added competition in Europe. As for me, while I do like McKesson over the long run, I would probably suggest waiting for more concrete statements from each company before chasing its share price higher.

Finally, despite no company-specific news, Xcel Energy (NYSE: XEL) advanced by 1.5% on the day to take the third-biggest gain within the S&P 500. The thing to remember about awful market days is that defensive stocks that have lagged to the upside often shine. Xcel Energy is an electric utility operating in the North and Midwest, meaning it offers its investors fairly consistent cash flow regardless of the day-to-day gyrations of the stock market. Furthermore, its 4.1% yield has to be looking particularly attractive after a day like today.

Six stocks our co-founder wants you to know about
Tired of not seeing your holdings on the list of the day's best gainers? Motley Fool co-founder David Gardner, founder of the world's No. 1 growth-stock newsletter, has developed a unique strategy for uncovering truly wealth-changing stock picks. And he wants to share it, along with a few of his favorite growth stock superstars, with you! It's a special 100% free report called "6 Picks for Ultimate Growth." So stop settling for index-hugging gains, and click here for instant access to a whole new game plan of stock picks to help power your portfolio.

Fool contributor Sean Williams has no material interest in any companies mentioned in this article. You can follow him on CAPS under the screen name TMFUltraLong, track every pick he makes under the screen name TrackUltraLong, and check him out on Twitter, where he goes by the handle @TMFUltraLong.

The Motley Fool owns shares of, and recommends Facebook, LinkedIn, and Monster Beverage. It also recommends McKesson and Pandora Media. Try any of our Foolish newsletter services free for 30 days. We Fools don't all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.