Google Calendar update adds Dark Mode on Android3
announcement contains a couple of screenshots showing the both Calendar and Keep apps with dark mode enabled. If you want to start using it on Google Calendar, you should head to Settings / General / Theme, and select Dark mode. It's much easier to enable it in Google Keep; simply go to Settings and choose Enable Dark Mode.
Now, it's worth mentioning that dark mode for Calendar is only supported on device running Android Nougat and higher. Also, if you have Android Q preview installed on your phone, you'll be able to set the entire OS in dark mode by default. The same goes for Google Keep with one exception, dark mode will work on all devices powered by Android Lollipop and later.
According to Google, the update that brings dark mode to Calendar and Keep apps will be rolled out to compatible Android devices over the course of 15 days. The rollout has already begun on May 16 for Google Calendar, while Google Keep will get it starting today, May 20.