This modification allows a global discount (of either a fixed price or a percentage) to be applied automatically to a user's shopping cart, between any two given dates.

No file edits are required!

[Premium version available: http://www.opencart.com/index.php?route=extension/extension/info&extension_id=9136]
In the premium version you are able to specifiy certain categories to provide a discount in, and add tiered discounting systems (i.e spend over £500, get 10% off. Spend over £250, get 5% off).
The premium version also allows you to set a long list of discounts active between different dates.