For weekly log files, the name is changed every 7 days (on the 7th, 14th,
etc). One thing that might be confusing  is that the log won't be created
until there is  some traffic on the  list. So, if there is  no message on
the 7th, the LOGyymmB file might not  begin until the 8th, which can give
the impression that the selection is random.
 
  Eric