Hi Mukesh,

There are several coupled approaches to achieve this. To help you I am providing to single approach using schedular. Please take help from this code and do necessary changes:

global class sendGreetingMsg implements Schedulable {
   global void execute(SchedulableContext SC) {
      //I trust your org have only 100 users
      //Customize logic based on date of birth
      List<User> users= [select id,Name,Email,DOB__c from user where DOB__c=today];

      // Step 0: Create a master list to hold the emails we'll send
      List<Messaging.SingleEmailMessage> mails = 
      new List<Messaging.SingleEmailMessage>();
      
      for (User myUser : users) {
        if (myUser.Email != null ) {
          // Step 1: Create a new Email
          Messaging.SingleEmailMessage mail = 
          new Messaging.SingleEmailMessage();
        
          // Step 2: Set list of people who should get the email
          List<String> sendTo = new List<String>();
          sendTo.add(myUser.Email);
          mail.setToAddresses(sendTo);
        
          // Step 3: Set who the email is sent from
          mail.setReplyTo('no-reply@gmail.com');
          mail.setSenderDisplayName('Sender Name');
        
          // (Optional) Set list of people who should be CC'ed
          List<String> ccTo = new List<String>();
          ccTo.add('anyotherccemail@gmal.com');
          mail.setCcAddresses(ccTo);

          // Step 4. Set email contents - you can use variables!
          mail.setSubject('Greeting Message');
          String body = 'Dear ' + myUser.Name + ', ';
          body += 'Provide your greeting message here';
          mail.setHtmlBody(body);
        
          // Step 5. Add your email to the master list
          mails.add(mail);
        }
      }
      // Step 6: Send all emails in the master list
      Messaging.sendEmail(mails);
   }
}

Note: Do not forget to schedule this class.


Thanks
Gulshan Raj