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.

The stock market brought an early Christmas to investors today, as major indexes, including the benchmark S&P 500 Index (SNPINDEX:^GSPC), hit all-time highs. Last week was a pivotal period for markets: The Federal Reserve finally decided to trim its monthly asset-buying program and GDP estimates were revised higher, to a 4.1% growth rate in the third quarter. The unabashedly bullish year continued into the holidays today, as the S&P added 9 points, or 0.5%, to end at 1,827.

Michael Kors (NYSE:KORS) stock ended as the biggest decliner in the entire 500-stock index, falling 3.7% Monday. With little company-specific news on the wires today, the sell-off was largely unsubstantiated, more likely the result of an underwhelming retail environment over the weekend than any issue with the fashion company. Michael Kors shareholders should be able to afford a high-end handbag or two after the stock's 58% run this year; Kors hopes to compete head-to-head with established industry leaders like Coach heading into the future. 

Shares of Micron Technology (NASDAQ:MU), the $22 billion Idaho-based chipmaker, slumped 3.1% today, after Merrill Lynch downgraded the stock and bestowed it with the dreaded "underperform" rating. Average selling prices, or ASPs, for Micron's DRAM chips are expected to fall as competition heats up and supply increases. Micron stock suffered a sudden 4.8% drop last Wednesday on rumors of South Korean chipmaker Hynix's plans to build a new factory in 2014.

Although Walgreen (NASDAQ:WBA) stock surged on Friday, shares in the drugstore lost 2.5% on Monday, as investors decided they weren't really so crazy about the company's fiscal first-quarter report last week after all. Despite technically beating estimates, margins were criticized as weak, and when adjusting for one-time gains, Walgreen failed to match expectations. The loss stood in stark contrast to rival Rite Aid (NYSE:RAD), which soared 5% today as it continued its remarkable year.

Fool contributor John Divine has no position in any stocks mentioned. You can follow him on Twitter, @divinebizkid, and on Motley Fool CAPS, @TMFDivine.

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