Use unorthodox date format in minical title

// We might use this approach if a standard PHP date format doesn't
// quite match what we want. Example: "Sept" for September (neither
// "F" or "M"  formats get us there).

// Change the mini cal title
add_filter('tribe_events_the_mini_calendar_title', 'change_minical_title_month_format');

// Look for "Sep" and change it to "Sept"
function change_minical_title_month_format($title) {
    return str_replace('Sep', 'Sept', $title);
}