docs: public void setSkipDays(final List<String> skipDays) {
          if (skipDays != null) {
              for (int i = 0; i < skipDays.size(); i++) {
                  String day = skipDays.get(i);
                  if (day != null) {
                      day = day.toLowerCase();
                      if (!DAYS.contains(day)) {
                          throw new IllegalArgumentException("Invalid day [" + day + "]");
                      }
                      skipDays.set(i, day);
                  } else {
                      throw new IllegalArgumentException("Invalid day [null]");
                  }
              }
          }
          this.skipDays = skipDays;
      }