I have developed new windows service that sends SMS to some numbers, and put timer but I need to know how can i customize it to run only every Friday and saturday every 3 hours . my code as below

public partial class IF_CHECK_SERVICE : ServiceBase
    {
        SqlConnection con = new SqlConnection("my connection details");
        System.Timers.Timer objTmr = new System.Timers.Timer();
        string value = "GCS Interface Jobs Run Successfuly are ";
        string value2 = "GCS Interface Jobs have Problem";
        string SMS = "";
        public IF_CHECK_SERVICE()
        {
            InitializeComponent();
        }

        protected override void OnStart(string[] args)
        {

            objTmr.Interval = 10800000;
            objTmr.Enabled = true;
            objTmr.AutoReset = true;
            objTmr.Elapsed += new System.Timers.ElapsedEventHandler(objTmr_Elapsed);
           
            
        }
        private void objTmr_Elapsed(object sender, System.Timers.ElapsedEventArgs e)
        {
           // TimeSpan objTime = new TimeSpan(12, 0, 0);
            //if (TimeSpan.Compare(DateTime.Now.TimeOfDay, objTime) >= 0)
            //{
                send();
            //}
        }

        protected override void OnStop()
        {
            //RBRTimer.Stop();
            //RBRTimer.Enabled = false;
        }
        void send()
        {
            try
            {

                if (con.State == ConnectionState.Open)
                    con.Close();
                string outErrorMessage = string.Empty;
                string messageStatus = string.Empty;
                System.Data.SqlClient.SqlDataAdapter ada = new System.Data.SqlClient.SqlDataAdapter("select JOB_ID , isnull(LAST_END_DATETIME,Getdate()) [Last_run],DATEDIFF(hh,isnull(LAST_END_DATETIME,getdate()),GETDATE()) DIFF from T_SCHEDULE_MASTER_USG105 where JOB_ID in ('SPA','PCO','WBR','PSIS','PSIR','WBR','KISR') order by DIFF", con);
                con.Open();
                DataSet ds = new DataSet();
                ada.Fill(ds);
                DataTable tbl = ds.Tables[0];
                for (int i = 0; i < tbl.Rows.Count; i++)
                {

                    DataRow myRow = tbl.Rows[i];
                    string JOB_ID = myRow["JOB_ID"].ToString();
                    int diff = int.Parse(myRow["DIFF"].ToString());
                    string last_run_time = myRow["Last_run"].ToString();
                    if (diff < 2)
                    {
                        value += JOB_ID + "\r\n";
                       // label3.Text = value;
                        SMS = value;
                    }
                    if (diff >= 2)
                    {
                        value2 += JOB_ID + "\r\n";
                       // label1.Text = value2;
                        SMS += "\r\n" + value2 + "Last Running Time " + last_run_time;
                    }

                }
               //Send SMS

                con.Close();

            }
            catch { }
        }

       
    }