It is generally not a good investment tactic to time the market. But in today's volatile environment, investors are sometimes tempted to buy stocks back after they have sold them at a loss. The general rule is that you cannot recognize the loss if substantially the same security is repurchased 30 days before or 30 days after--within a 61 calendar day range--the sale.
The disallowed loss is added to the basis of the repurchase security.
Here is a good link for the rules:
http://www.fairmark.com/capgain/wash/index.htm
