A See the Page documentation.

N Special Price. The special price feature let's you set a price different from the standard price of the product, usually for a limited period of time. On the product page the special price displays in the right sidebar, in bold, below the regular price. In category and search grids, the special price displays below the regular price under the product image.

To set or remove a special price. Go to Catalog | Manage Products. Use the search boxes to find your product. Choose View for the product you want to edit. Choose the Prices tab on the left side. Enter a value in the Special Price field, set a beginning date in the Special Price From Date and an end date in the Special Price To Date. You can set these up in advance, if you wish, so that a special price will take effect on some future date.

When done, use the Save button in the top right.