Hello,

Thank you for your question.

I have made a simple WebCalender sample to select all Sunday and Saturday on this month.
I add some validation javascript code in OnAfterRender client side event.
I check all dates in this month and add all Sunday and Saturday to SelectedDates collection.
Here the example snippet javascript code, how I select all Sunday and Saturday:

function OnAfterRender(controlId, calendarType) {    var WebCalendar1 = ISGetObject(controlId);
    var calendar = WebCalendar1.Calendars[0];
    var month = calendar.Month;
    var year = calendar.Year;
    sundays(year, (month - 1));
    return true;
}
function sundays(year, month) {
    var WebCalendar1 = ISGetObject("WebCalendar1");
    WebCalendar1.SelectedDates.Clear();
    var day = 1;
    var counter = 0;
    var date = new Date(year, month, day);
    while (date.getMonth() === month) {
        if (date.getDay() === 0 || date.getDay() === 6) { // Sun=0, Mon=1, Tue=2, etc.
            //To add the date to SelectedDate collection
            WebCalendar1.SelectedDates.Add(new WebCalendarSelectedDates(date));
            WebCalendar1.SelectedDate = date;
        }
        day++;
        date = new Date(year, month, day);
    }
}

Please kindly also have reviews on my sample to see the result and let me know your response.

Regards,
Hans.