Hi! Thanks for your interest in this subject.
You can find description of the plugin interface when you download plugin sample
Just look at comments inside.
This plugin is written with Visual C++ in Microsoft Visual Studio Express 2013.
You can use it as a base of your own plugin for Ultimate Calendar.
If you have any questions about plugin and have irresistible desire to write plugin for Ultimate Calendar, please don't hesitate to contact me at support(a)maximus.in.ua or here on this forum.
And if you have any ideas to make plugin code more correct or useful or maybe you want to see any plugin interface expansion just contact me with your proposals. I will be very grateful.
When you will have plugin as ready to use, it can be placed on the UC website or even in the UC distributive.
PS. Some ideas for plugin: women calendar; weather; phrase of the day; I don't smoke for x days...