I use one from Fdroid. The appl-ess alternative would have been to add colored days to my Google Calendar, but I already have too much shit there and I don’t need more clutter. In particular I don’t want Google having more of my data.

I also don’t want my cycles exposed if I’m showing my calendar in person to someone.

Periodical, the app I chose, lets me enter the dates, the intensity, it forecasts my upcoming two cycles, and keeps a log of all my previous cycles in text format that is very easy to read. It’s super useful when going to the doctor; instead of “uhhh, I think I’ve been regular the past six months or so” I can just pull out a list of dates and if I’m really looking for details I have a log of symptoms too.