http://www.financialpost.com/Make+crash+with+loss+selling/1062990/story.html

An older article but correct I think:

<snip>

But you need to know the rules.”

The main one is the superficial loss rule. If you want to stay invested, you can’t just sell Stock X, declare a loss and repurchase it a day later. (You can sell it at a gain and repurchase right away, but not with a loss.) The Canada Revenue Agency would disallow your tax loss on the grounds it was a “superficial” loss. The rule is you must wait 31 days before repurchasing the identical security.

<snip>