columnFormat 1.3

Determines the text that will be displayed on the calendar’s column headings.

{
  month: 'ddd',    // Mon
  week: 'ddd M/d', // Mon 9/7
  day: 'dddd M/d'  // Monday 9/7
}

A single string will set the title format for all views. A View Option Hash may be provided to target specific views (this is what the default does).

Uses formatDate/formatDates formatting rules. (The formatting rules were much different in versions before 1.3. See here)