This way
<asp:GridView ID="GridView1" runat="server" AutoGenerateColumns="false" >
<Columns>
<asp:BoundField DataField="Day1" />
<asp:BoundField DataField="Day2" />
<asp:BoundField DataField="Day3" />
<asp:BoundField DataField="Day4" />
<asp:BoundField DataField="Day5" />
<asp:BoundField DataField="Day6" />
<asp:BoundField DataField="Day7" />
</Columns>
</asp:GridView>
protected void Page_Load(object sender, EventArgs e)
{
DataTable dt = new DataTable();
dt.Columns.AddRange(new DataColumn[7] {
new DataColumn("Day1"),
new DataColumn("Day2"),
new DataColumn("Day3"),
new DataColumn("Day4"),
new DataColumn("Day5"),
new DataColumn("Day6"),
new DataColumn("Day7"),
});
dt.Rows.Add();
GridView1.DataSource = dt;
GridView1.DataBind();
for (int i = 0; i < GridView1.HeaderRow.Cells.Count; i++)
{
GridView1.HeaderRow.Cells[i].Text = DateTime.Now.AddDays(i).ToShortDateString();
}
}