A widget I like(d) oddly much in the Notification Centre is the Tomorrow Summary, because it was a good finishing round-up to the Calendar widget. When upgrading to macOS Sierra a while back, the new update did not affect this widget. However, upgrading to iOS 10, I cannot seem to find this widget anymore. Has it been removed? Is it possible to somehow get a summary like this to show up in the Notification Centre?
The "Tomorrow" built-in widget has been removed from iOS 10.
You could use the calendar app Fantastical. It contains a widget which can display a calendar view of the current month, with a list of events for each day, and you can tap around on the calendar to change the day being viewed without ever leaving the widget. It would require an additional tap versus the Tomorrow view, and it's not a summary so much as an actual list, but it's a decent alternative for your use case (and a great app in general, in my opinion).