property Event.BackColor as Color
Retrieves or sets a value that indicates the date event's background color.

TypeDescription
Color A color expression that indicates the date event's background color. The last 7 bits in the high significant byte of the color to indicates the identifier of the skin being used. Use the Add method to add new skins to the control. If you need to remove the skin appearance from a part of the control you need to reset the last 7 bits in the high significant byte of the color being applied to the background's part.

Use the Events property to access the Events collection. Use the Background property to change the visual appearance for parts in the control. Use the Background(exDropDownAppearance) property to change the visual appearance of the drop down portion of the control. Use the Background(exDropDownBackColor) property to specify the background color of the drop down portion of the control. The BackgroundExt property provides unlimited options to show additional colors, text, icons, images, frames, patterns, ... to any event. 

The following sample shows how to bold the "tomorrow" date:

Calendar1.Events.Add(Date() + 1).Bold = True