Can not really help without seeing the SQL for the report. Here is the likely problem, but I can not tell you where it is. Somewhere in the query or the sorting and grouping is this function
Weekday(date, [firstdayofweek])
Where date is your field. In your query it would look like this
Weekday([yourFieldName])
Because the default is Sunday.
If you can find this function change to
Weekday([yourFieldName],vbMonday)
This sets to Monday.
This function returns a number 1 to 7. So if you put in a date that falls on monday and you put vbMonday as the second parameter it returns a 1 and a day that falls on Sunday returns a 7. But if you do not put in the second parameter it returns a 1 for Sunday and a 2 for Monday