Static Constructor:

A static constructor is used to initialize static data only once, or used to perform action only once.

It will be executed only once, unlike the normal constructor that is executed whenever the object is created.

Syntax:

    class SimpleClass
    {
        // Static constructor
        static SimpleClass()
        {
            //...
        }
    }

Points to remember:

1.  Static Constructor cannot be called directly.
2.   No access modifiers allowed with Static Constructor.
3.   Parameters not allowed in Static Constructor.
4.   A static constructor is called automatically to initialize the class before the first instance is created or any static members are referenced.
5.  The user has no control on when the static constructor is executed in the program.
6.  The static constructor for a class executes before any instance of the class is created.
7.  Static constructor executes only one time.

Example:

In following example constructor EmployeeLeave checks weather current day is working day or week end day. If current day is Saturday or Sunday constructor sets static field value WorkingDay to false so Employee is not eligible to apply leave.  

public class EmployeeLeave
    {
        private static bool isWorkingDay;
 
        public string EmployeeID {get;set;}
        public decimal remaningLeaves{get;set;}
        public DateTime FromDate {get;set;}
        public DateTime TodateDate {get;set;}
 
        static EmployeeLeave()
        {
            //Getting the Current Date using DateTime Class 
            DateTime currentDate = DateTime.Now; 
 
            //Checking the Weekend as it is Saturday or Sunday 
            if(currentDate.DayOfWeek==DayOfWeek.Saturday|| currentDate.DayOfWeek==DayOfWeek.Sunday) 
            { 
                //If the Current Date is Saturday or Sunday 
                //consider as holiday  
                isWorkingDay =false; 
            } 
             
            //Days Other than Weekends(i.e other than Staurday or Sunday) 
            else 
            { 
                //If it is not weekend 
                //Consider working day
                isWorkingDay = true; 
            }            
        }
 
        public void LeaveApplication()
        {
            if(isWorkingDay==true)
            {
                //leave application Code
            }
            else
            {
                //Warning message ,tody is saturday or sunday
            }
        }
    }