diff --git a/c3l_membership/forms.py b/c3l_membership/forms.py
--- a/c3l_membership/forms.py
+++ b/c3l_membership/forms.py
@@ -57,6 +57,8 @@ class MembershipForm(Form):
         ],
     )
 
+    birthday = DateField("Birthday")
+
     street = StringField(
         "Nr., Street",
         validators=[Length(max=4000)],