code: private Map<String, Integer> parseFirstDays(JsonObject root) {
      JsonObject node = resolve(root, "supplemental", "weekData", "firstDay");
      Map<String, Integer> m = new HashMap<>();
      for (String key : objectKeys(node)) {
        String value = string(node, key);
        m.put(key, NUMERIC_DAYS.get(value));
      }
      return m;
    }
docs: Parse the "first day" values. This is the value of the first day of the week
  in a calendar view.