All the main controls (Calendar, Scheduler, Month) require that you change the visible date range in the code explicitly using StartDate and Days properties. A typical example is the Command event fired automatically by the Navigator control (see also Demo/Calendar/Default.aspx.cs, line 282 in the DayPilot package). That means you should be aware of any changes. You can use these properties (StartDate, Days) to set the limits to your SQL query.
The only exception is the Dynamic Scheduler control, which loads the data dynamically during scrolling. After each scroll position change, a Scroll event handler is fired. All the information about the visible area are available in ViewPort property (e.g. DynamicScheduler1.ViewPort.Start, DynamicScheduler1.ViewPort.End).