How to return specific dates based on multiple conditions in SQL?

How about..

where
    (reportDate between @YTD_Start_Dt and DATEADD(day, -8, @report_dt) and periodtype  = 'mth') or
    (reportDate between DATEADD(day, -6, @report_dt) and @report_dt and periodtype  = 'wk') or
    (DAY(@report_dt) < 8 AND periodtype  = 'wk' and reportDate between DATEADD(d, -6, EOMONTH(@report_dt, -1)) AND EOMONTH(@n, -1))

.. where it's a

• mth type and the record is between year start and 8 days prior (removes the latest mth if it's the first week of the month after) or
• wk type in the last week or
• it's the first week of the month and is a wk report from the ultimate 7 days of last month