What happened

Shares of e-commerce powerhouse Amazon (AMZN 3.43%) were up about 2% at 3:30 p.m. ET on Friday. The jump outpaced the broad market's gain and you could credit investors' optimistic response to Amazon's announced plans for this year's Prime Day online sale. The above-average bounce, however, may have taken shape even without the announcement that was made around 1 a.m. on Thursday.

So what

Not that investors didn't know it was coming sooner or later, but it's arguable that yesterday's official notification of this year's Prime Day sale was the tangible catalyst investors needed to start scooping up the stock today. The two-day event scheduled for July 12 and 13 is not only a reward for existing Prime subscribers, but also a means of attracting new paying members. Prime customers tend to spend more at Amazon.com than non-Prime customers -- on the order of two to four times as much, according to estimates -- so the company wants to sign up members.

Don't read too much into Friday's bounce, though. Amazon stock ended Thursday's session more than 39% below its late-March peak. This steep sell-off left the stock ripe for bullishness. And take note of the fact that while the Prime Day news is exciting, the market didn't respond to it the same day it was released. Today's marketwide bounce may actually be the true bullish catalyst.

Now what

While Prime Day may look like the most obvious prompt for Friday's gains, veteran investors know it's not really about the sale. The announcement of Prime Day is merely a justification for buying a well-loved, high-performance stock that's been beaten so far down in just a few weeks. Investors often need specific developments to answer the "why buy it now?" question. Prime Day is an adequate answer for many.

It's this very same psychological dynamic, however, that should be prompting would-be buyers to think twice before jumping in. We've seen equally bullish headlines take shape over the course of the past couple of months, and while some of them sparked a bullish jolt for the stock, all of those jolts lacked any real follow-through. There's no assurance this one will result in a different outcome, particularly if the broad market is still in a bigger-picture downtrend.

That's not to suggest Amazon shares can't continue rallying from here; they may well do so. It's simply to say today's gain is more about the stock's usual volatility and less about the company's likely e-commerce revenue and income prospects.

In other words, don't go chasing one day's good performance.