js implements calendar to get the week number and day of the week for a specified date. Example sharing (js gets the day of the week)_Basic knowledge

function onCheck(){
var Year = document .getElementById("year").value; //Get the "year" of the text box var theYear =Year * 1; //Convert to number type //alert(theYear); //Get the month value
var month = document.getElementById("month");
var index1=month.selectedIndex; var theMonth = month.options[index1].value; //Get the monthly value
var day = document.getElementById("day") ;
var index2=day.selectedIndex;
var theDay = day.options[index2].value;

//Input value judgment part
...
//Call core function
days(theYear,theMonth,theDay);
}

function days(year,month,day) {
var days = 0; //Indicates the day of the year when the date is changed
//Add the number of days in the month
for(var i = 1; i < month; i ){
switch(i) {
//Add 31 to the big moon
case 1:
case 3:
case 5:
case 7:
case 8:
case 10:
case 12:{
days = 31;
break;
}
//In Xiaoyue’s case, add 30
case 4:
case 6:
case 9:
case 11:{
days = 30;
break;
}
//For February, add
according to the year type case 2:{
if( isLeapYear(year)){
days = 29; //Add 29 for leap years
}
days = 28;
}
break;
}
}
}
day = day * 1;
days = day; //The sum of the number of days in months plus the number of days in days

var date0 = new Date(year,0,1); //What day of the week is the first day of the year?

// alert(date0.getDay());
var date1 = new Date(year ,month-1,day); //Format the date value, 0-11 represents January-December;
// alert((days date0.getDay() 6)/7);
var nthOfWeek = Math.floor((days date0.getDay() 6)/7); // Round down
// alert(nthOfWeek);
var toDay = new Array("Sunday","week "1", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday");
//day.getDay(); Returns a day of the week based on Date, where 0 is Sunday
alert("This date is the " days " of the year " " It is the " nthOfWeek " week " toDay[date1.getDay()]);
}

if (isNaN(theYear)|| theYear < 0 ) {
alert("Incorrect input, please re-enter");
return ;
}

if((theMonth == 2 && theDay > 29 && isLeapYear(theYear))||(theMonth == 2 && theDay > 28 && !isLeapYear(theYear))) {

alert("Enter Yes Wrong, please re-enter");
return ;
}

if((theMonth == 4 || theMonth == 6 || theMonth == 9 || theMonth == 11) && theDay == 31 ) {

alert("Incorrect input, please re-enter" );
return ;
}