Try this...
string j1 = Convert.ToString(days1);
att = "";
ad = new SqlDataAdapter("select D" + j1 + " from Attendance where Emp_Code='" + TextBox1.Text + "' and reg='" + regn1 + "' and [Type]='" + Label112.Text + "'", con);
DataTable dt6 = new DataTable();
ad.Fill(dt6);
if (dt6.Rows.Count > 0)
{
att = Convert.ToString(dt6.Rows[0][0]);
}