It's a modified plugin by Seditio.by, ported additionally to Siena.
It displays a calendar in page lists, in which are shown pages created in given date in this category and its subcategories. It also contains a bigger calendar.
Quite useful in blog setups.
Check tpl folder for additional css. Hope you like it.
http://www.cotonti.com/datas/users/calendar_253.zip