For Visual Cron
Below C# code returns true or false if today is first day of the month or not.
using System;
public class FirstDayMonth
{
    public static bool FirstDayOfMonth()
 {
    DateTime now = DateTime.Now; 
    
    
    var firstDay = new DateTime(now.Year, now.Month, 1);
   
    if (now.Day == firstDay.Day)
  
        return true ;            
    else
        return false ;
 }
}
Last working day:
using System;
public class LastDayMonth
{
    public static bool LastDayOfMonth()
 {
    DateTime now = DateTime.Now; 
    
    int daysInMonth = DateTime.DaysInMonth (now.Year, now.Month);
    
    var lastDay  = new DateTime(now.Year, now.Month, daysInMonth);
   
    if (now.Day == lastDay.Day)
  
        return true ;            
    else
        return false ;
 }
}