How to require at least one attribute

I'm selling a food item that can be picked up on four different dates. It costs $5.00 per item, and customers can choose one or more of four dates. How do I set up attributes to require at least one of the dates to be selected and to have the final price reflect the number of dates they've chosen (eg, if they choose three dates, the price at checkout will say $15, but if they choose only one date, it will be $5.00)?

Thanks.

I'm still having trouble setting this up. Here's what I want to do:

-- The customer decides to purchase the item
-- The item is $5.00
-- They can pick up the item on one or more dates (up to four); if they select one of the four dates, it will be $5.00 total, but if they select three of the four, it will be $15.00 total

How do I set up the Attributes Controller to require the customer to select one of the dates and allow them to select up to four of the dates? I've read through a lot of forum entries and still can't figure out how to do this.

Have you an URL to this Product so we can get a better idea of what you are doing now? :lookaroun