По совпадению, я сделал нечто подобное несколько лет назад:
https://github.com/Subterrane/calendar
Этот пример кода не самый красивый (перенесенный из Typescript), но, по сути, вы правы.Найдите день, установите цвет.
"use strict";
document.addEventListener("DOMContentLoaded", function(event) {
var calendars = document.getElementsByTagName('calendar');
for (var ix = 0; ix < calendars.length; ix++) {
var calendarEl = calendars[ix];
var now = new Date();
var y = parseInt(calendarEl.getAttribute('year')) || now.getFullYear();
var m = parseInt(calendarEl.getAttribute('month')) || now.getMonth() + 1;
var d = parseInt(calendarEl.getAttribute('day')) || now.getDate();
var time = new Date(y, m - 1, d);
var month = time.getMonth();
var week = 1;
var day = 1;
var html = "<year number=\"" + y + "\"><month number=\"" + m + "\"><week number=\"" + week + "\">";
time.setDate(day);
do {
if (time.getDay() % 7 === 0)
html += "</week><week number=\"" + ++week + "\">";
html += "<day number=\"" + day + "\"></day>";
time.setDate(++day);
} while (month === time.getMonth());
html += '</week></month>';
calendarEl.innerHTML = html;
calendarEl.querySelector('day[number="' + d + '"]').classList.add('today');
}
var days = document.querySelectorAll('day');
for (var ix = 0; ix < days.length; ix++) {
days[ix].addEventListener('click', dayClicked);
}
});
function dayClicked() {
var el = this;
var data = {
year: el.parentElement.parentElement.parentElement.getAttribute('number'),
month: el.parentElement.parentElement.getAttribute('number'),
week: el.parentElement.getAttribute('number'),
day: el.getAttribute('number')
};
var event = new CustomEvent('calendarClicked', {
'detail': data
});
document.dispatchEvent(event);
}
calendar {
display: block;
margin-top: 20px;
max-width: 600px;
font-family: sans-serif;
}
week {
display: flex;
}
week:first-child {
justify-content: flex-end;
}
day {
flex-basis: 14.3%;
box-sizing: border-box;
background-color: gainsboro;
border: 1px solid white;
padding: 2%;
color: white;
cursor: pointer;
font-size: 4vw;
}
day::after {
content: attr(number);
}
.today {
background-color: gold;
}
<div class="container">
<calendar></calendar>
</div>