CalendarMenu is a macOS menu bar application, which provides a small calendar showing all information you need to keep track of all upcoming appointments, birthdays and reminders in the blink of an eye.
CalendarMenu is highly configurable and allows you to specify exactly what you want CalendarMenu to show.
On macOS Big Sur, a Today Widget for the Notification Center is available showing a monthly calendar in three different styles, with markers for all your appointments, reminders and birthdays (see the 3rd screenshot).
On the previous macOS versions like macOS Catalina and earlier, two more list widgets for your upcoming events (today and tomorrow) or reminders are available as well (see the 4th screenshot)
Updated on Mar 10 2023
Please note: the Widget Extension on macOS Big Sur and later still requires the main app (the one residing in the menu bar) to be running in order to be updated properly, since these kinds of widgets aren't technically able to update themselves all on their own.