Google has been hard at work building out support for dark mode across apps such as YouTube, Contacts, Messages, and Android as a whole, and finally, that work has extended to the Google Calendar and Google Keep apps according to updates noted on the G Suite blog.
The changes initially started rolling out on May 16, but Google notes it'll be a gradual rollout so if you do not see the changes yet you'll have to hang tight until Google enables it across the board. They note it can take up to 15 days.
In any case, you'll want to make sure you have Google Calendar, and Google Keep updated. Once updated you can enable dark mode by going to Settings > General > Theme within Calendar and Settings > Enable Dark Mode within Keep.