When setting up a scheduled data import, there is a check box called Conditional that allows you to enter a script. There is a wiki article here: http://wiki.servicenow.com/index.php?title=Scheduling%5FData%5FImports

What the wiki article fails to do (at the time of writing this), is give an example of what variable to return as true/false. Like other places within the system, you need to set answer to true for the scheduled import to run and false if you don't want it to run.

I have an example in the image below. It runs every 5 minutes except on Sunday.