We've written many times about how unrealistic it is to think that you can profit by timing the market consistently and correctly. After all, let's face it -- from day to day, it's kind of impossible to know what the market is going to do.

According to data by Davis Advisors Funds, between 1993 and 2007, the S&P 500 averaged an annual gain of about 10.5%. But if you'd been out of the market on just the 10 best days, your average gain would have dropped to merely 7.1%. Miss the top 30 days, and your gain drops to 2.2% annually. To see these in clearer context, consider that at 10.5%, a $10,000 investment will grow to $44,700 over 15 years. At 7.1%, it will become just $28,000, fully 37% less.

I was reminded of this lesson yesterday, when the S&P 500 surged 7.1% in just one day! Yes, this was one of those days that matter in an investing lifetime.

Individual stocks, too ...
I was also reminded of the lesson a week or two ago, when I was thinking of snapping up a few more shares of Warren Buffett's Berkshire Hathaway (NYSE:BRK-A) for my portfolio. On a day when I was mulling it over, the company's B shares shot up a heart-stopping $445 per share! Yes, Berkshire stock costs a lot per share to begin with, but still, that was a gain of almost 20% in a single day. And remember, we're talking about an insurance company that also sells candy and underwear, not a fiber-optics start-up or a technology outfit taking China by storm.

So did I buy into the company the next day? Nope. I waited for a better price, again. And as I type this, the stock is up a further 7%, having gained that much just yesterday. Sigh.

I'm sure that others are feeling my pain. If you have been thinking about buying any of the following stocks, for example, check out how they fared on just one day, March 23, 2009. (I'll also include their star rating from our Motley Fool CAPS community, in case you're interested in researching any of them further -- and note that Berkshire, my original target, enjoys a five-star rating, out of five.)

Company

CAPS rating (5 stars max.)

Mar. 23 one-day gain

Amazon.com (NASDAQ:AMZN)

**

8.0%

CarMax (NYSE:KMX)

***

11.2%

Home Depot (NYSE:HD)

**

4.9%

Washington Post (NYSE:WPO)

**

8.8%

Comcast (NASDAQ:CMCSA)

**

10.7%

JPMorgan Chase

**

24.7%

Alcoa (NYSE:AA)

****

13.2%

Data: Yahoo! Finance, Motley Fool CAPS.

What to do
One of the lessons in this is that it's not always smart to wait.

Do wait, though, if you haven't yet done enough research into a stock. Do wait if you don't have sufficient confidence in its promise. Do wait if you think it's overvalued right now, or perhaps just fairly valued. In such cases, your money will be more likely to grow elsewhere, where you have more confidence, where you're more familiar with the company, where the stock seems significantly undervalued.

Don't wait, though, if you've concluded that a company is one you really want in your portfolio. Don't wait if the current price is attractive, offering a good shot at growth. Don't pass up a compelling, undervalued stock in the hope of getting an even better price for it -- unless you're willing to lose out on it altogether. Because days like yesterday can happen, and they can bump you out of a stock that was trading at an attractive price. (Fortunately, though, many big gains are partially or completely erased within short order, only to be duplicated and eventually surpassed, later.)

If you'd like any pointers to some compelling and seemingly rather undervalued stocks, I encourage you to test-drive our Motley Fool Inside Value newsletter. A free trial will give you access to all past issues and all recommended stocks. Remember, recessions are often the best time to buy.

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Longtime Fool contributor Selena Maranjian owns shares of Berkshire Hathaway, Home Depot, and CarMax. Berkshire Hathaway, Home Depot, and CarMax are Motley Fool Inside Value picks. Amazon.com and Berkshire Hathaway are Motley Fool Stock Advisor recommendations. JPMorgan Chase is a former Motley Fool Income Investor selection. The Fool owns shares of Berkshire Hathaway. Try our investing newsletters free for 30 days. The Motley Fool is Fools writing for Fools.