diff --git a/happening.py b/happening.py
index c25307ef..1c81e0d5 100644
--- a/happening.py
+++ b/happening.py
@@ -49,6 +49,8 @@ def saveEventPost(baseDir: str, handle: str, postId: str,
     If an event has extra fields, as per Mobilizon,
     Then it is saved as a separate entity and added to the
     events timeline
+    See https://framagit.org/framasoft/mobilizon/-/blob/
+    master/lib/federation/activity_stream/converter/event.ex
     """
     calendarPath = baseDir + '/accounts/' + handle + '/calendar'
     if not os.path.isdir(calendarPath):
diff --git a/inbox.py b/inbox.py
index d7c1baf7..1721105f 100644
--- a/inbox.py
+++ b/inbox.py
@@ -772,6 +772,8 @@ def receiveUndo(session, baseDir: str, httpPrefix: str,
 
 def isEventPost(messageJson: {}) -> bool:
     """Is the given post a mobilizon-type event activity?
+    See https://framagit.org/framasoft/mobilizon/-/blob/
+    master/lib/federation/activity_stream/converter/event.ex
     """
     if not messageJson.get('id'):
         return False
@@ -804,6 +806,8 @@ def receiveEventPost(recentPostsCache: {}, session, baseDir: str,
                      personCache: {}, messageJson: {}, federationList: [],
                      nickname: str, debug: bool) -> bool:
     """Receive a mobilizon-type event activity
+    See https://framagit.org/framasoft/mobilizon/-/blob/
+    master/lib/federation/activity_stream/converter/event.ex
     """
     if not isEventPost(messageJson):
         return