 Simple Calendar app is a great, free and open source application available in FDroid for your Android device. The fault theme is ugly, but you can change that. But there is one issue, by default, if you have your phone set to do not disturb, your notifications still come through, which can be annoying if they go through in the middle of the night. If you go into your do not disturb settings and look at see all exceptions, you'll notice by default on most Android device, allow alarms is checked, which is good, because you want your alarms to wake you up if you're sleeping, that's usually why you set them. But I don't necessarily want my calendar notifications waking me up. If I go back into Simple Calendar app, click on Settings and scroll down to the third section here, Event Reminders. You'll see that by default, the audio stream used is alarms. If you don't want your calendar notifications waking you up when your phone is set to silence, change it to something as notifications. I hope you found this useful and I hope that you have a great day.