The Shopify Virtual Assistant App lets you schedule adminstrative tasks. You choose the date and let the assistant take care of running the task.

Current tasks include:

• Increase or decrease the price of a product by a specific amount, or a percentage.
• Hide or show a product - perfect for daily deal sites (works with a countdown timer too).
• Add or remove a product from a collection.
• Change product type.
• Synchronize inventory for product bundles.

7 day trial - Sign up here: http://www.shopifyassistant.com