Special Strips
A few times there have been a special strip inside a story.
The X-Mas strips
In 1934, 1935 and 1937 one could find a special X-mas strip in the dailies for December 25th. One find a similar strip for The Phantom in 1938.
The Defender of the Earth strip
The Earth Day strips
In 2008 and 2009 one could find a special Earth Day strip in the dailies for April 22nd.