A Red Letter Day is a day of special significance within the calendar such as public holidays.
To add a Red Letter Day, in your navigation > TIMESHEETS > RED LETTER DAYS and click on the 'Add Red Letter Day' button or to edit an existing Red Letter Day in your list, click on the pencil icon to the left of the name of the day in your list.
Pick the date of the Red Letter Day, give it a name, colour and tick wether the date is a public holiday or not.
Click submit to add it to your list of Red Letter Days
Now when you view the date in your Activity or Schedule, the entire day will be coloured in the colour you specified in the Red Letter Day settings.