diff --git a/calendarweek/calendarweek.py b/calendarweek/calendarweek.py
index 516f041f99ed0ccf539e5074b6422c96cb908017..f02acf7b3395c5b3a37c1890d1eafe9b88476260 100644
--- a/calendarweek/calendarweek.py
+++ b/calendarweek/calendarweek.py
@@ -125,7 +125,7 @@ class CalendarWeek:
         return datetime.strptime("%d-%d-%d" % (self.year, self.week, n + 1), "%G-%V-%u").date()
 
     def __contains__(self, day: date) -> bool:
-        return self.__class__.form_date(day) == self
+        return self.__class__.from_date(day) == self
 
     def __eq__(self, other: CalendarWeek) -> bool:
         return self.year == other.year and self.week == other.week