Which backups are kept as weekly or monthly backups?

The backups between Friday night and Saturday morning are chosen as weekly backups. Monthly backups are those done between the last night of a month and the first morning of the following month.

For example, the weekly backup for the week starting 4 Mar, 2012 was taken between 9:00pm, 9 Mar and 7:00am, 10 Mar. The monthly backup of Mar 2012 was taken between 9:00pm, 31 Mar and 7:00am, 1 Apr.