The KeepSake message is a private message sent from a guest to the host, and no one else can view it. This is an opportunity for a guest to post a heartfelt message from the moment, that can be cherished at a later time by just the host.
The host sets the date it will be displayed in the Event's timeline, prior to the Event.
For a guest to upload a KeepSake message, click the add '+' button and select 'KeepSake'. Then click the 'Photo' or 'Video' button. Select a photo/video, enter a message, and click 'Post'. The popup is displayed as shown below.