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

In the last trading session of 2013, the stock market bade a fitting farewell to a successful year, as major market benchmarks posted gains of around half a percent. Amid the generally positive mood, many stocks posted even more significant gains, with Twitter (NYSE:TWTR), Acceleron Pharma (NASDAQ:XLRN), and Hertz Global Holdings (NYSE:HTZ) among the best stocks of the day.

Twitter gained 5% to close the year up 145% from its $26 IPO price. Today's bounce came after a couple of days of big declines that cut more than 17% off the social-media company's stock price. Yet even despite concerns that LinkedIn (NYSE:LNKD.DL) and Pinterest have more people using their services than Twitter, bullish investors remain convinced in the microblogging site's long-term potential to monetize its user base effectively and become profitable in due course.

Acceleron Pharma continued its recent rise, adding another 12% Tuesday. The biotech company became a member of the Russell 2000 index on Dec. 20, and since then, shares have soared more than 30% as investors become more familiar with the company. Acceleron develops protein therapeutics to help treat cancer and other rare diseases, and more importantly, the company has the backing of biotech giant Celgene (NASDAQ:CELG), which held a stake of about 11% in the company as of September according to S&P Capital IQ. With Acceleron having received a $7 million from Celgene as a milestone payment for starting a phase 2 trial of its sotatercept treatment for end-stage renal disease, biotech investors are starting to look at the small company as an up-and-comer in the industry.

Hertz Global Holdings climbed more than 10% after it adopted a poison-pill defense to thwart any potential hostile bid for the car-rental giant. The jump might seem counterintuitive, given that Hertz's move makes it harder for a potential buyout to occur. Yet investors clearly hope that the poison pill reflects an unknown party's interest in the company, and even if a would-be acquirer has to jump through extra hoops, it could still result in a bid that gives shareholders a big gain on their investment.