SQL Help - loop through dates

Trying to create a query which will retrieve information and store in a temp table so I can report on the values.  The report needs to show the information by each Friday.  I am creating a temp table which adds each Friday by date

Like this.

reportdate                      source      item_no      quantity      trx_dt      promise_dt
2013-06-14 00:00:00.000      NULL      NULL      NULL      NULL      NULL
2013-06-21 00:00:00.000      NULL      NULL      NULL      NULL      NULL
2013-06-28 00:00:00.000      NULL      NULL      NULL      NULL      NULL

However, my other data doesn't fall into each Friday order.

source      item_no                                        quantity      doc_type      trx_dt
P      ITK2236                             60.0000      O      2013-06-14 00:00:00.000
O      ITK2236                             14.0000      A      2013-06-14 00:00:00.000
P      ITK2236                             60.0000      O      2013-06-17 00:00:00.000
P      ITK2236                             60.0000      O      2013-06-19 00:00:00.000
O      ITK2236                             15.0000      A      2013-06-21 00:00:00.000
O      ITK2236                             27.0000      A      2013-07-26 00:00:00.000


How can I loop, cursor, whatever through these records and sum the values of the quantity by the trx_dt by the appropriate Friday.  This will only run by 1 item_no at a time, but there could be muliple records per trx_dt.

Any help would be appreciated?