Cycle confusion!

wishingonastar

Hmm this is such a silly questions but my apps are confusing me! I have a 26 day cycle, so is my period due on the 26th day of that cycle or the 27th day of you see what I mean! I’m trying to figure out if I’m due tomorrow or Sunday, this app says Sunday and my other app says tomorrow 🤔 I’m on cycle day 25 now.