rotating logs

Hi

I need to rotate a log file but without having a frequency, i.e. I
wish to trigger a log rotation ONCE only after some external event; how
can I do this?
renaming the file is one option, however I don't wish to kill and
restart the httpd server. But the way I understand it rotatelogs is
just working when you assign a frequency, so it may be out of phase
with the trigerring event next time.