Access 97- Reports grouped by Date

I have developed a database in Ms Access 97. I have designed reports with dates which are grouped by week. However, the details and calculations on the reports are not accurate since I want the week to Start on a Monday and end on a Sunday. The system has a default week which starts on a Sunday and ends on a Saturday.

Please can you assist me as to how I can change my reports so that the week starts on a Monday and ends on a Sunday.