What is the easiest way to do free shipping for a limited time?

Use a shipping override to offer free shipping for a limited time.

Holidays are a great time to offer a special or discount. Free shipping can be accomodated a few ways. You can create a code to discount the shipping a percentage or dollar amount but that requires your customer to enter a code during checkout. The easiest way to offer free shipping is to setup a shipping scale override. 

Access this option by going to Settings>Shipping/Handling>Click on your scale>Add A Shipping Override. 

You can set it up to show up for a certain date range (ex: from thanksgiving through cyber Monday) or for a certain offset from your event date (ex: first 48 hours after your event). You can also setup the shipping fees to change as the dollar amount goes up. (ex. free shipping after they spend $40 on products) The example below is for free shipping for the first two days above $40. Be sure to click save after you have made the changes. You will need to refresh at least one event using that shipping scale for the override to take effect.