Twelve Month Calendar is a simple, free calendar app that displays the entire year on one screen. It is designed to provide a no-frills, basic calendar experience, with options to customize the display. The current date is highlighted with a blue background, and US holidays are marked in red. Sundays are also displayed in red.

The app offers preferences for choosing Sunday or Monday as the first day of the week, displaying week numbers, highlighting holidays in red, displaying the days countdown to a specific date, and highlighting today's date. The app displays the following US holidays: New Year's Day, Martin Luther King Jr's Birthday, George Washington's Birthday, Memorial Day, Juneteenth National Independence Day, Independence Day, Labor Day, Columbus Day, Veterans Day, Thanksgiving Day, and Christmas Day.

When the app is launched, the current year is displayed. Users can swipe to change the displayed year or enter a year via the keyboard. The background changes to blue when a year other than the current year is displayed.

Overall, Twelve Month Calendar is a straightforward calendar app that provides a clear and simple way to view the entire year at once. It may not have all the bells and whistles of more complex calendar apps, but it gets the job done.