Fertile window

​I have a few questions. I'm currently using glow and a fertility app to track, my fertile days. According to glow I was fertile was week, but then the other app shows I'm fertile this week. I'm confuse! Can someone explain more the fertility window? I normally have a 31 day cycle my two past period have been on the 25th ( meaning I had my period on December 25th and then again on January 25). Can someone tell me how to determine my fertility window?

​ Regards, 

​Michelle:)