Normally, you would handle the CellDrawn event for the grid to try to decorate a cell with custom drawing after the cell is drawn. But, with the ScheduleControl, the underlying grids come and go as you change dates or change viewtypes, etc. So, it is difficult to keep a particular grid event hooked up as the grids get destroyed and recreated.
So, a better solution is to derive the ScheduleGrid and override OnCellDrawn to handle your custom drawing code. To get your ScheduleControl to use the derived grid, you also have to derive the ScheduleControl and override CreateScheduleGrid so that an instance of your derived grid is used.
Here is the recurrence sample that we ship modified to show how you might do this. The sample will draw a little red square on any non-allday appointment for Day, WorkWeek and Custom views.
http://www.syncfusion.com/support/user/uploads/CS_e4081491.zip