Hi,
This is fairly easy to accomplish.
You could use grep.
First copy your logfile to a *.old filename.
$ cp logfile logfile.old
Then grep -v to get rid of undesirable lines containing the pattern you want to delete.
$ cat logfile.old |grep -v "Nov/2001" > logfile
OR
Better, you can just save your logs to *.old1, *.old2 when the first day of the month arrives, run from cron.
# minutes (0-59) hours (0-23) dates (1-31) months (1-12) days (0-6)
0 0 1 * * /some/log/cleahup.sh
$ vi cleanup.sh
cp /some/log/file1 /some/log/file2
cp /some/log/file /some/log/file1
> /some/log/file