Looking to have a calendar which I can use with Linux and Android

Hello people,

I hope this question is not too nooby. What I am looking for is a calendar which I can use in my Notification center and for e.g. Evolution/Thunderbird and with an app on Android where I can see my appointments on the widgets. But I want to leave Google services and don’t know what I have to look for, but would love to have a open source solution of course!
Can you guys help me out or suggest me sth?

PS: A huge bonus would be to add appointments which i get through Outlook.