MedHelp has a great app called MyCycles if you want to download
it. (tool bar at the top, apps section). day one of your period
is cycle day one. Count from there. Last day of your cycle is the day before your next one starts. You'll probably eventually notice a pattern. Around the holiday time in particular though, many women are irregular because that can happen with changes in routine, stress, weight gain or loss, illness, etc. which are all very common during holiday time.