I have a piclist called Account type . Whenever the value changes, I need to store the day in another field. 

I am creating isExecuting Scenario. I had written trigger on Account and This is Helper class below... 
public class AccountNumberUpdate {
    public void checkUpdate()
     {
         if(Trigger.isExecuting)
         {
             for(Account accountRecordNew : Trigger.new)
             {
             for(Account acountRecordOld: Trigger.old)
             {
                if(acountRecordNew.id == acountRecordOld)
                   //logic
             }
         }
     }
     }
}

I am creating isExecuting Scenario. I had written trigger on Account and This is Helper class below... 
public class AccountNumberUpdate {
    public void checkUpdate()
     {
         if(Trigger.isExecuting)
         {
             for(Account accountRecordNew : Trigger.new)
             {
             for(Account acountRecordOld: Trigger.old)
             {
                if(acountRecordNew.id == acountRecordOld)
                   //logic
             }
         }
     }
     }
}