Hi,
You will need to load your data into memory first, then handle the DatePicker's DayRender event based on whatever information you have in your memory (in theory you can also do a database query whenever DayRender is called, However that would be inefficient because DayRender is called for each day cell). The second part has nothing to do with your database.
There is no simple "syntax" to do so. It needs to be done by code. Here is a sample demonstrating how to handle DayRender event:
http://demo.essentialobjects.com/Demos/Calendar/Features/Scheduler/Demo.aspxNote the "holidays" are in 2009 and 2010 (Hasn't been updated for 2011 holidays yet). So you will need to scroll back to see result.
Thanks