Comments by Chinger (Top 1 by date)

reportGridView is a GridView variable. It had the value of MonthToDateGrid when it's being populated initially

Originally set:

var dataAdapter = new SqlDataAdapter(command);
var dataTable = new DataTable();
dataAdapter.Fill(dataTable);

reportGridview.DataSource = dataTable;
reportGridview.DataBind();
ReportHeader.Text = reportTitle;

Page Change:
protected void MonthToDateGrid_PageIndexChanging(object sender, GridViewPageEventArgs e)
{

MonthToDateGrid.PageIndex = e.NewPageIndex;
MonthToDateGrid.DataBind();
}