"Day starts at" and "Day ends at" are applied to day and week calendars when using "horizontal" week calendar.
You can choose the type of week and month calendars, as well as their starting weekday.
You can also choose your default calendar and default reminder list, which will be used when you create a new event or reminder. You can also set the default duration for events.
You can customize the color of the title bar or selection in tab bar. You can also specify the background color for calendars. If you are using dark mode in iOS, the colors you specify here will not be applied.
You can also specify dark mode or light mode within the app. For example, the system can use the dark mode while the app runs in light mode. Also, you can customize the colors that you can apply to days from the popup menu.
Weekends & Holidays:
You can choose to customize the foreground (text) color and the background color for Saturdays, Sundays and holidays.
You can change the font size & style (bold), which will be applied to all calendars and the list view.
You can also select how each type of event (e.g., timed, all-day, multi-day) is rendered. You can also choose to apply this to month calendar only. On iPhone, you can also choose to show time in month calendar.
You can turn on the option to show the location of an event along with its title. You can also select what type of icon(s) to display for events.
You can choose whether the event viewer (which shows the detail of event when you tap on it) is displayed in dark or light colors.
You can choose the default alerts for timed and all-day events and reminders.
You can also choose to show alerts on birthdays and anniversaries registered in "Contacts" app.
You can select (or unselect) the calendars to show (or hide) in the app.
You can create, edit or delete calendars by tapping the "Edit" button at the top-right corner.
When you turn on "Rollover" option, over-due reminders will be moved to today's date.
You can also choose to show or hide completed reminders. You also have an option to delete completed reminders to improve performance.
You can also specify which calendar to show in the reminders view.
The new widgets in iOS 14 is fully supported. You can customize its appearance (text and background colors). You can also specify which calendars to show in the widgets
The legacy widget in iOS 13 and earlier will be deprecated in the future. You can choose the style of widget from day, 7-days, week or month views. When you turn on "Show date bar" option, you can change the date in the widget.