You've answered your own question. Have a form pop up that prompts for the dates. In the query criteria, reference the input boxes on the form (the form must be open when the query is run). Use the expression builder to help you if neccessary.
EDIT: Aw, heck. How about I be super nice?
Between [Forms]![frmDateInput]![txtDateRangeStart] And [Forms]![frmDateInput]![txtDateRangeEnd]
That's the criteria expression for your Date in the query, where frmDateInput is your popup form and txtDateRangeStart/End are the two input boxes. You can name them whatever you want, of course.