The headerDateProperty property (string) specifies the date format pattern that will be used by the JavaScript Calendar component to format the dates in columns headers.
If the value is not specified the datePattern string from the current locale will will be used.
The pattern uses syntax of DayPilot.Date.toString(pattern) method.
DayPilotnullCalendar config:
{
headerDateFormat: "dddd MMMM d, yyyy", // produces "Thursday January 1, 2035"
// ...
}