Reassuring comments from Federal Reserve Chairwoman Janet Yellen helped the S&P 500 (^GSPC -0.46%) race to a 0.8% gain on the last day of March. The index closed at just over 1,872, capping off a so-so month for the markets. Still, it's been a great run for the S&P 500 after the downbeat start to the year: Since early February, the index has picked up more than 7% and has investors feeling much more confident about the market's momentum.

Speaking in Chicago today, Yellen cited ongoing weakness in the American economic picture even with the recent recovery in the jobs market. Investors inferred that Yellen's reference to weak wage growth and declining labor force participation, among other matters, allows for plenty more room for the Fed to continue its post-recession history of low interest rates.

We'll get a much better picture of just how well the economy is rolling along come Friday, when the Department of Labor is set to release the jobs report for March. Economists on average predict a gain of about 200,000 jobs this month, which would improve substantially on February's 175,000-job gain.

Still, the S&P 500 and the market's biggest gainers weren't looking too far ahead today. American Capital (ACAS) shares jumped higher by 11.2% on a day of few huge movers on the market, while biotech stock Alnylam Pharmaceuticals (ALNY -0.06%) climbed 9.7%. Among larger-cap stocks, Micron Techology (MU -0.18%) shares gained more than 7.5%.

American Capital shareholders announced that it had repurchased 8.9 million of its shares in the first quarter; more importantly, the business developer is halting its share repurchase program. The stock has shed more than 1% over the past year despite the market's great gains. As fellow Fool analyst Jordan Wathen notes, American Capital's buyback plan hasn't helped the company trade at a higher value, and its lack of a dividend puts it a disadvantage against the competition. Ending a plan that hasn't paid off and pursuing a more aggressive course of maximizing its value could be the company's next step from here.

Alnylam bounced back today in a relief to investors who have weathered a terrible month from this developmental biotech stock.

Despite a huge gain over the past year, Alnylam's shares have collapsed by more than 22% in the past month. That hasn't waved off interest from Big Pharma: Just this past week, Sanofi's (SNY 5.90%) Genzyme unit invested a further $23 million into the company, bolstering its original $700 million investment. Alnylam's clinical-stage transthyretin-mediated amyloidosis treatment patisiran is the drug furthest along in the company's pipeline, having entered phase 3 trials last November after scoring strong phase 2 results. It'll be a while before any conclusive findings emerge from late-stage studies, but keep an eye on the drug: J.P. Morgan projected in January that patisiran, if approved, could evenually rake in $700 million in peak sales annually.

Finally, Micron hit a high today among bigger-name stocks ahead of its earnings release this Thursday. While Micron made news for being added to the growth stock index Barron's 400, investors likely are looking forward to the company's results. Analysts project a big turnaround from last year, when the company reported a $0.28 per-share loss in the quarter ending Feb. 14. Analysts on average project earnings per share of $0.74 for this quarter. If Micron can ward off concerns over the possibility of falling prices in the memory market and capitalize on demand from the mobile market, it could surprise investors.