Um, yes. The *most* fertile time is a week or so after her period ends. The least fertile time is during the period. But that doesn't mean much. 
"Abstinence on specific days of the menstrual cycle remains a method of family planning for many couples worldwide.18 Women should be aware that no calendar method is completely effective. Our data suggest there are few days in the menstrual cycle during which some women are not potentially capable of becoming pregnant---including even the cycle day on which they may expect their next menses to begin. "
from 
http://bmj.bmjjournals.com/cgi/conte.../321/7271/1259