Advice on keeping track of my period?

I'm trying to keep track of my period so I can have a better idea of when it'll come next month. Up until this point I had a general idea, but just kinda went with fhe flow (no pun intended, lol). Now I really want to keep records of what day it starts and how long it lasts, but I have a question. My period came last night at around 10:30. I'm not sure if I should count that as the "first day" though. Technically I did get it on June 26, but there was only an hour and a half left of the day. I'm getting all the typical first day symptoms today, whereas I didn't have any last night. Should I count today, June 27th, as the first day, or yesterday?