Look at AutoPostBack
] and OnChange of your Auction Date control.
Basically you need to attach to the OnChange event of the Auction Date Control, when that changes you just need to query the database and pull out the details for the given date and populate your list box like you normally would.
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Oh yeah, when you are in your OnChange event you can access the value of what ever control you are using to select the auction date by looking at the corresponding property for that control