Most likely your custom content type is deriving from Item
content type. You need to inherit it from Event
content type. If you are adding a content type using Visual Studio, you are presented with an option to select the parent content type as shown:

However, if you already have created it, you can still modify the Elements.xml file. Id of Event
is 0x0102
. Append this value in ID property of Content Type followed by the GUID. Example:
?xml version="1.0" encoding="utf-8"?>
<Elements xmlns="http://schemas.microsoft.com/sharepoint/">
<!-- Parent ContentType: Event (0x0102) -->
<ContentType ID="0x010200B5BDA8DA0B8944F0AE8ACE1A38228A59"
Name="Holiday"
Group="Custom Content Types"
Description="My Content Type"
Inherits="TRUE"
Version="0">
<FieldRefs>
</FieldRefs>
</ContentType>
</Elements>
Event
content type.