Store week number calculation's first day of week

src/Calendar.js

@@ -332,6 +332,47 @@ function Calendar_constructor(element, overrides) {
 		}
 	})(options.weekNumberCalculation);
 
+	// Store the first day of the week according to the week number
+	// *calculation* method, not the week *display*.
+	// With ISO: Always 1 (Monday). Is not influenced by the firstDay
+	//   option.
+	// With custom function: Test a sequence of 7 days with the custom
+	//   function to determine on which day of the week the week number
+	//   changes. Is not influenced by the firstDay option.
+	// With local: Set to the value of localeData._week.dow, which will
+	//   contain the value of firstDay, or else will return the dow value
+	//   from a protype, such as the locale or moment's Locale object
+	//   default 0 (Sunday).
+
+	localeData._fullCalendar_weekCalcFirstDoW = (function(weekCalc, localeDoW) {
+		var weekCalcFirstDoW;
+		if (weekCalc === 'ISO') {
+			weekCalcFirstDoW = 1;
+		}
+		else if (typeof weekCalc === 'function') {
+			var m;
+			var wkNr;
+			var wkNrPrev;
+			// Test for first day of week. Start comparing with Sunday (0).
+			m = moment.utc("2015-01-04T00:00Z");
+			wkNrPrev = weekCalc(m);
+			weekCalcFirstDoW = 0; // Assume Sunday.
+			// Check Monday through Saturday for change in week number.
+			for (i = 1; i < 7; i++) {
+				m.add(1, 'days');
+				wkNr = weekCalc(m);
+				if (wkNr != wkNrPrev) {
+					weekCalcFirstDoW = i;
+					break;
+				}
+				wkNrPrev = wkNr;
+			}
+		}
+		else if (weekCalc === 'local') {
+			weekCalcFirstDoW = localeDoW;
+		}
+		return weekCalcFirstDoW;
+	})(localeData._fullCalendar_weekCalc, localeData._week.dow);
 
 
 	// Calendar-specific Date Utilities