Unfortunately this is not possible yet. You can however specify 1 pixel for the header height and remove the header text. Here is the code for this:

calendar1.TimetableSettings.HeaderDateFormat = "  ";
calendar1.TimetableSettings.MainHeaderSize = 1; 

Note: Pay attention that the format of the header text is two spaces. It seems that DateTime.Format gives errors when trying to use only one space as a formatting string.

This way the header will look like a thin line. You can alse set the color of that line through the HeaderBrush property of the appropriate Style object. The followig code sets the color to White.

calendar1.TimetableSettings.Style.HeaderBrush =
    new MindFusion.Drawing.SolidBrush(Color.White); 

Meppy