well, if you set the header fields to the max of the corresponding results in the query, you would get the ACTUAL start and end dates. If you need the actual parameters you put in, I don't know how you'd do it from a parameter query.
To solve that problem, I created global variables, opened a form in the OnOpen event of the report (the call to the form MUST incluce the instruction to open in dialog mode - acDialog or something). Then, in the click event of a command button on the form, set the globals equal to the values in the textboxes of the form, and close the form. You need to create two functions to read the globals. set the parameters of the report driving query equal to the functions that read the form. also set the header fields equal to the functions.
the result will be a call to the report which opens the form. you enter the dates in the form and click the button. the values will be transferred to the globals, the form will close and the the report will finish opening using the the parameters you entered.
Advantages, you can see if you've entered the parameters correctly before you push the button and you are only asked once. You cluld also include code for validation purposes.