Hi vail,

Refer the below sample. I have not tested the code. So you need to implement and test the code.

For showing popup i have used the below article.

Open (Show) ASP.Net AJAX ModalPopupExtender Modal Popup from Code Behind (Server Side) in ASP.Net

Here i have used timer to refresh the gridview in every 1 hour to check the expiry date. Now the code executed as per your condition.

So you need to change the code as per your requirement.

HTML

<div>
    <asp:ScriptManager runat="server" />
    <asp:Timer ID="Timer1" runat="server" OnTick="Timer1_Tick" Interval="1000">
    </asp:Timer>
    <asp:GridView runat="server" ID="gvDocuments" AutoGenerateColumns="false" OnRowDataBound="gvDocuments_RowDataBound">
        <Columns>
            <asp:BoundField DataField="DocumentID" HeaderText="ID" />
            <asp:BoundField DataField="DocumentName" HeaderText="Name" />
            <asp:BoundField DataField="DocumentExpiryDate" HeaderText="Expiry Date" />
        </Columns>
    </asp:GridView>
    <asp:LinkButton ID="lnkDummy" runat="server"></asp:LinkButton>
    <cc1:ModalPopupExtender ID="ModalPopupExtender1" BehaviorID="mpe" runat="server"
        PopupControlID="pnlPopup" TargetControlID="lnkDummy" BackgroundCssClass="modalBackground"
        CancelControlID="btnHide">
    </cc1:ModalPopupExtender>
    <asp:Panel ID="pnlPopup" runat="server" CssClass="modalPopup" Style="display: none">
        <div class="header">
            Modal Popup
        </div>
        <div class="body">
            <asp:Panel runat="server" ID="pnlStatus">
            </asp:Panel>
            <br />
            <asp:Button ID="btnHide" runat="server" Text="Hide Modal Popup" />
        </div>
    </asp:Panel>
</div>

C#

protected void Page_Load(object sender, EventArgs e)
{
    if (!this.IsPostBack)
    {
        BindGrid();
    }
}

private void BindGrid()
{
    DataTable dt = new DataTable();
    dt.Columns.AddRange(new DataColumn[3] { new DataColumn("DocumentID", typeof(int)), new DataColumn("DocumentName"), new DataColumn("DocumentExpiryDate") });
    dt.Rows.Add(1, "Test1.doc", "10/1/2017");
    gvDocuments.DataSource = dt;
    gvDocuments.DataBind();
}

protected void Timer1_Tick(object sender, EventArgs e)
{
    Timer1.Interval = ((1000 * 60) * 60); //1 Hour
    BindGrid();
}

protected void gvDocuments_RowDataBound(object sender, GridViewRowEventArgs e)
{
    if (e.Row.RowType == DataControlRowType.DataRow)
    {
        Label lbl = new Label();
        DataTable expiredDocuments = new DataTable();
        expiredDocuments.Columns.AddRange(new DataColumn[3] { new DataColumn("DocumentID", typeof(int)), new DataColumn("DocumentName"), new DataColumn("DocumentExpiryDate") });
        DateTime expiryDate = Convert.ToDateTime(e.Row.Cells[2].Text);
        DateTime currentDate = DateTime.Now;
        if (Convert.ToDateTime(currentDate.ToShortDateString()) <= Convert.ToDateTime(expiryDate.AddMonths(-3).ToShortDateString()))
        {
            lbl.Text = "Document expires after 3 months";
            lbl.ForeColor = System.Drawing.Color.Yellow;
            pnlStatus.Controls.Add(lbl);
            ModalPopupExtender1.Show();
        }
        else if (Convert.ToDateTime(currentDate.ToShortDateString()) <= Convert.ToDateTime(expiryDate.AddMonths(-2).ToShortDateString()))
        {
            lbl.Text = "Document expires after 2 months";
            lbl.ForeColor = System.Drawing.Color.Orange;
            pnlStatus.Controls.Add(lbl);
            ModalPopupExtender1.Show();
        }
        else if (Convert.ToDateTime(currentDate.ToShortDateString()) <= Convert.ToDateTime(expiryDate.AddMonths(-1).ToShortDateString()))
        {
            lbl.Text = "Document expires after 1 months";
            lbl.ForeColor = System.Drawing.Color.Red;
            pnlStatus.Controls.Add(lbl);
            ModalPopupExtender1.Show();
        }
    }
}