As you see today it is the second of april but it wont show me the right selected day.
This is a link to the project. https://github.com/dragosreal1214/Calendar_Site_FIIPractic
use today.getDate() instead of today.getDay().
today.getDay() returns of the day of the week (0-6) of a date.
if(i.toString() === today.getDate().toString() &&
today.getMonth() === currDate.getMonth() && today.getFullYear() === currDate.getFullYear()) {
day.setAttribute("class", "today");
}