Ti ringrazio innanzitutto per il tuo prezioso aiuto!
Io ho tentato di spiegarmi... ma sicuramente l'ho fatto male!
Questo è il codice della pagina dove ho problemi, intorno alla riga 34...
Come ho detto nel post precedente con l'input di tipo radio funziona... a me serve che funzioni senza radio, ma con un hidden.. in modo che il submit del form avvenga direttamente cliccando sulla data.
Ecco il codice della pagina (calendar.php3):
Codice PHP:
<html>
<head>
<title>Untitled Document</title>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
<script Language="JavaScript">

function Calendar_GetString()
{
    var firstDate = new Date(this.year,this.month,1);
    var firstDay = firstDate.getDay();
    var monthArg = this.month + 1;
    var calStr = "<form method=POST name="+Calendar.monthName[this.month].toUpperCase()+" action=calendar.php3><input type=hidden name=chiave value=teschio><TABLE BORDER=1 COLS=7>";
    calStr += "<TR>";
    calStr += "<TD COLSPAN=7 ALIGN=center><SPAN class=caldefault><font size=2>"+Calendar.monthName[this.month].toUpperCase()+" "+this.year+"</font></SPAN></TD><input type=hidden name=mese value="+monthArg+"><input type=hidden name=anno value="+this.year+">";
    calStr += "</TR>";
    calStr += "<TR>";

    for (var i=0;i<Calendar.dayName.length;i++) 
        calStr += "<TD ALIGN=center><SPAN class=caldefault><font size=2>"+Calendar.dayName[i]+"</font></SPAN></TD>";
    calStr += "</TR>";

    var dayCount = 1;
    calStr += "<TR>";
    for (var i=0;i<firstDay;i++) 
        calStr += "<TD> </TD>";


    for (var i=0;i<this.monthDays[this.month];i++)
    {
        var styleStr = "calday";
        if (dayCount==this.date)
            styleStr = "caltoday";
            
        calStr += '<TD ALIGN="center"><input type=radio name=giorno value='+
                    dayCount+' onClick="document.'+Calendar.monthName[this.month].toUpperCase()+'.submit();">'+
                    '<SPAN class="'+styleStr+'"><font size=2>'+
                    dayCount+'</font></SPAN></A></FONT></TD>';
        dayCount++;
        if ((i+firstDay+1)%7==0&&(dayCount<this.monthDays[this.month]+1)) 
            calStr += "</TR><TR>";
    }

    var totCells = firstDay+this.monthDays[this.month];
    for (var i=0;i<(totCells>28?(totCells>35?42:35):28)-totCells;i++) 
        calStr += "<TD> </TD>"
    calStr += "</TR>";
    calStr += "</TABLE></form>";
    return calStr;
}



function Calendar_Show()
{
    var calStr = this.GetString();
    document.write(calStr);
}

function Calendar(date,month,year,action)
{
    // Properties
    this.date = date;
    this.month = month;
    this.year = year;
    this.action = action;
    this.monthDays = new Array(31,28,31,30,31,30,31,31,30,31,30,31);

    // Leap year test
    if ((this.year%4==0||this.year%100==0)&&(this.year%400==0)) 
        this.monthDays[1] = 29; 
    else 
        this.monthDays[1] = 28;
        
    // Methods
    this.Show = Calendar_Show;
    this.GetString = Calendar_GetString;
}

Calendar.dayName = new Array("Dom","Lun","Mar","Mer","Gio","Ven","Sab");
Calendar.monthName = new Array("Gennaio","Febbraio","Marzo","Aprile","Maggio","Giugno","Luglio","Agosto","Settembre","Ottobre","Novembre","Dicembre");


function CalendarSet_Show()
{    
    var border = '';
    document.write('<table border="0" cellpadding="0" cellspacing="0"><tr>');

    // Show the top border
    for (var i=0;i<this.calendars.length+2;i++)
    {
        document.write(border);
    }
    document.write('</tr><tr>');

    document.write(border);
    for (var i=0;i<this.calendars.length;i++)
    {
        document.write('<td>');
        this.calendars[i].Show();
        document.write('</td>');
    }
    document.write(border);
    document.write('</tr><tr>');
    
    // Show the bottom border
    for (var i=0;i<this.calendars.length+2;i++)
    {
        document.write(border);
    }
    document.write('</tr></table>');
}


function CalendarSet_SetMonthOptions(selectField)
{
    var selected = true;
    selectField.options.length = 0; // Clear the popup
    
    for (var i=0;i<this.calendars.length;i++)
    {
        var anOption = new Option(Calendar.monthName[this.calendars[i].month]+" "+this.calendars[i].year,(this.calendars[i].month+1)+","+this.calendars[i].year,selected,selected);
        selectField.options[i] = anOption;
    
        selected = false;
    }
    selectField.options[0].selected = true;
}

function CalendarSet_SetDateOptions(monthSelectField,dateSelectField)
{
    var calObject = this.calendars[monthSelectField.selectedIndex];
    var daysInMonth = calObject.monthDays[calObject.month];
    
    if (dateSelectField.options.length==0)
    {
        var anOption = new Option("Date",0,true,true);
        dateSelectField.options[0] = anOption;    
    }
    
    if (dateSelectField.options.length > daysInMonth+1)
    {
        // Remove last entries
        var excess = dateSelectField.options.length - daysInMonth - 1;
        for (var i=0;i<excess;i++)
        {
            dateSelectField.options[dateSelectField.length-1] = null;
        }
    }
    else if (dateSelectField.options.length < daysInMonth+1)
    {
        // Add entries
        var deficit = daysInMonth - dateSelectField.options.length + 1;
        for (var i=0;i<deficit;i++)
        {
            var anOption = new Option(dateSelectField.options.length,dateSelectField.options.length,false,false);
            dateSelectField.options[dateSelectField.options.length] = anOption;
        }
    }
    

    var now = new Date();
    dateSelectField.options[now.getDate()].selected = true;
}

function CalendarSet(startDate,months,action)
{

    this.borderColor = "#42316B";
    this.calendars = new Array();

    var now = new Date();
    var month=startDate.getMonth();
    var year = startDate.getFullYear();
    
    for (var i=0;i<months;i++)
    {
        var dayToHilite = 0;
        if ((now.getFullYear()==year)&&(now.getMonth()==month))
            dayToHilite = now.getDate();
        this.calendars[i] = new Calendar(dayToHilite,month,year,action);
        month++;
        if (month>=12)
        {
            month = 0;
            year++;
        }
    }


    this.Show = CalendarSet_Show;
    this.SetMonthOptions = CalendarSet_SetMonthOptions;
    this.SetDateOptions = CalendarSet_SetDateOptions;
}

</script>
  
</head>

<body bgcolor="#FFFFFF" text="#000000" marginwidth="0" marginheight="0" leftmargin="0" topmargin="0">

<script Language="JavaScript">

function HandleCalClick(day,mon,year)
{
    
        document.write('" + mon + "/" + day + "/" + year');
}


var gImageDir = "images/"


var now=new Date();
var gCalendarSet = new CalendarSet(now,1,"HandleCalClick");

gCalendarSet.Show();

</script>
</body>
</html>