I think this code will help you:-
Suppose Date Column is in cell number 3:
string Day = e.Row.Cells[3].Text;
string Name = String.Empty;
switch (Day)
{
case "M":
Name = "Monday";
break;
case "T":
Name = "Tuesday";
break;
case "W":
Name = "Wednesday";
break;
case "Th":
Name = "Thursday";
break;
case "F":
Name = "Friday";
break;
case "S":
Name = "Tuesday";
break;
case "Su":
Name = "Saturday";
break;
}
e.Row.Cells[3].ToolTip = Name;
Good luck.