This is usually the date on which the batch was created. When you create a new batch, the session date is used by default as the batch date. However, you can change the batch date any time before you post the batch, provided the Ready To Post option is not selected.
The batch date is used as the default document date for new documents you add to the batch.
Once the batch is posted, the batch date becomes part of your audit trail. You use it to select batches to print on the batch listing and batch status reports.