Session_Start Event not firing daily for the first user

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C#

ASP.NET

I have a function written inside session start event for sending mails.For the first user of the day it sends mail and makes a flag=true in database.The next time for the new user or the same user it checks for the flag, if its true It doesn't send mails.This is how I want the function to perform.In simple words I want my web app to send a mail daily if I am the first user in current date. But I think its not firing everyday.If it fires everyday and my function is working fine then I should get mail everyday.I get mails after 2 or 3 days. Why so ? Should the function be written in session start itself? Any ideas or suggestions are welcome.

session start event:-

protected void Session_start()
{
 try
{
  //code to check if flag is true for today's date
  if(flag != true)
  {
    //get data from database and sent it in mail.
    //insert flag=true for current date
  }
}
 catch()
 {
 }
}