How to add a tooltip on mouse over on the Grid View Column Heading

In your code behind, create the method rowDataBound for the GridView and add the below code

protected void GridView1_RowDataBound(object sender, GridViewRowEventArgs e)
{
    if (e.Row.RowType == DataControlRowType.Header)
    {
        foreach (TableCell cell in e.Row.Cells)
        {
            cell.Attributes.Add("title", "Tooltip text for " + cell.Text);
        }
    }
}

Don't forget to set the attribute OnRowDataBound in the GridView.

http://rosshawkins.net/archive/2007/04/15/adding-tooltips-to-gridview-headers.html.aspx

protected void grd_popup_details_RowDataBound(object sender, GridViewRowEventArgs e)
    {
        for (int i = 0; i < e.Row.Cells.Count; i++)
        {
            e.Row.Cells[i].ToolTip = e.Row.Cells[i].Text;
        }
    }

Refference link

I've never done any asp.net development, but there seems to be a solution provided here: how to add title for every header column in gridview in ASP.NET

your sample could look like this:

 <asp:GridView ID="grdView" runat="server" Width="900px" AutoGenerateColumns="False"
            AllowPaging="true" AllowSorting="true" CellSpacing="0" CellPadding="5" PageSize="20"
    OnRowDataBound="grdView_RowDataBound">
            <Columns>
 <asp:TemplateField HeaderText="ID Number" ItemStyle-Width="90px" >
<HeaderTemplate>
       <asp:Label ID="Header" ToolTip="HERE WE GO!!!!" runat="server" Text="Label"></asp:Label>
       </HeaderTemplate>
    <ItemTemplate>
      <asp:Label ID="Label1" runat="server" Text='<%# Bind("ID")%'></asp:Label>
    </ItemTemplate>
 </asp:TemplateField><asp:BoundField DataField="StudentName" HeaderText="StudentName"> </asp:BoundField>

I would give that a try :)