entonces te posteo el codigo entero.
solo con copiarlo ya funciona...
Código PHP:
var datePickerDivID = "datepicker";
var iFrameDivID = "datepickeriframe";
var dayArrayShort = new Array('Do','Lu', 'Ma', 'Mi', 'Ju', 'Vi', 'Sa');
var dayArrayMed = new Array('Dom','Lun', 'Mar', 'Mie', 'Jue', 'Vie', 'Sab');
var dayArrayLong = new Array('Domingo','Lunes', 'Martes', 'Miercoles', 'Jueves', 'Viernes', 'Sabado');
var monthArrayShort = new Array('Ene', 'Feb', 'Mar', 'Abr', 'May', 'Jun', 'Jul', 'Ago', 'Sep', 'Oct', 'Nov', 'Dic');
var monthArrayMed = new Array('Ene', 'Feb', 'Mar', 'Abr', 'May', 'Jun', 'Jul', 'Ago', 'Sept', 'Oct', 'Nov', 'Dec');
var monthArrayLong = new Array('Enere', 'Febrero', 'Marzo', 'Abril', 'Mayo', 'Junio', 'Julio', 'Agosto', 'Septiembre', 'Octubre', 'Noviembre', 'Diciembre');
// these variables define the date formatting we're expecting and outputting.
// If you want to use a different format by default, change the defaultDateSeparator
// and defaultDateFormat variables either here or on your HTML page.
var defaultDateSeparator = "/"; // common values would be "/" or "."
var defaultDateFormat = "dmy" // valid values are "mdy", "dmy", and "ymd"
var dateSeparator = defaultDateSeparator;
var dateFormat = defaultDateFormat;
function displayDatePicker(dateFieldName, displayBelowThisObject, dtFormat, dtSep)
{
var targetDateField = document.getElementsByName (dateFieldName).item(0);
// if we weren't told what node to display the datepicker beneath, just display it
// beneath the date field we're updating
if (!displayBelowThisObject)
displayBelowThisObject = targetDateField;
// if a date separator character was given, update the dateSeparator variable
if (dtSep)
dateSeparator = dtSep;
else
dateSeparator = defaultDateSeparator;
// if a date format was given, update the dateFormat variable
if (dtFormat)
dateFormat = dtFormat;
else
dateFormat = defaultDateFormat;
var x = displayBelowThisObject.offsetLeft;
var y = displayBelowThisObject.offsetTop + displayBelowThisObject.offsetHeight ;
// deal with elements inside tables and such
var parent = displayBelowThisObject;
while (parent.offsetParent) {
parent = parent.offsetParent;
x += parent.offsetLeft;
y += parent.offsetTop ;
}
drawDatePicker(targetDateField, x, y);
}
function drawDatePicker(targetDateField, x, y)
{
var dt = getFieldDate(targetDateField.value );
// the datepicker table will be drawn inside of a <div> with an ID defined by the
// global datePickerDivID variable. If such a div doesn't yet exist on the HTML
// document we're working with, add one.
if (!document.getElementById(datePickerDivID)) {
// don't use innerHTML to update the body, because it can cause global variables
// that are currently pointing to objects on the page to have bad references
//document.body.innerHTML += "<div id='" + datePickerDivID + "' class='dpDiv'></div>";
var newNode = document.createElement("div");
newNode.setAttribute("id", datePickerDivID);
newNode.setAttribute("class", "dpDiv");
newNode.setAttribute("style", "visibility: hidden;");
document.body.appendChild(newNode);
}
// move the datepicker div to the proper x,y coordinate and toggle the visiblity
var pickerDiv = document.getElementById(datePickerDivID);
pickerDiv.style.position = "absolute";
pickerDiv.style.left = x + "px";
pickerDiv.style.top = y + "px";
pickerDiv.style.visibility = (pickerDiv.style.visibility == "visible" ? "hidden" : "visible");
pickerDiv.style.display = (pickerDiv.style.display == "block" ? "none" : "block");
pickerDiv.style.zIndex = 10000;
// draw the datepicker table
refreshDatePicker(targetDateField.name, dt.getFullYear(), dt.getMonth(), dt.getDate());
}
/**
This is the function that actually draws the datepicker calendar.
*/
function refreshDatePicker(dateFieldName, year, month, day)
{
// if no arguments are passed, use today's date; otherwise, month and year
// are required (if a day is passed, it will be highlighted later)
var thisDay = new Date();
if ((month >= 0) && (year > 0)) {
thisDay = new Date(year, month, 1);
} else {
day = thisDay.getDate();
thisDay.setDate(1);
}
// the calendar will be drawn as a table
// you can customize the table elements with a global CSS style sheet,
// or by hardcoding style and formatting elements below
var crlf = "\r\n";
var TABLE = "<table cols=7 class='dpTable'>" + crlf;
var xTABLE = "</table>" + crlf;
var TR = "<tr class='dpTR'>";
var TR_title = "<tr class='dpTitleTR'>";
var TR_days = "<tr class='dpDayTR'>";
var TR_todaybutton = "<tr class='dpTodayButtonTR'>";
var xTR = "</tr>" + crlf;
var TD = "<td class='dpTD' onMouseOut='this.className=\"dpTD\";' onMouseOver=' this.className=\"dpTDHover\";' "; // leave this tag open, because we'll be adding an onClick event
var TD_title = "<td colspan=5 class='dpTitleTD'>";
var TD_buttons = "<td class='dpButtonTD'>";
var TD_todaybutton = "<td colspan=7 class='dpTodayButtonTD'>";
var TD_days = "<td class='dpDayTD'>";
var TD_selected = "<td class='dpDayHighlightTD' onMouseOut='this.className=\"dpDayHighlightTD\";' onMouseOver='this.className=\"dpTDHover\";' "; // leave this tag open, because we'll be adding an onClick event
var xTD = "</td>" + crlf;
var DIV_title = "<div class='dpTitleText'>";
var DIV_selected = "<div class='dpDayHighlight'>";
var xDIV = "</div>";
// start generating the code for the calendar table
var html = TABLE;
// this is the title bar, which displays the month and the buttons to
// go back to a previous month or forward to the next month
html += TR_title;
html += TD_buttons + getButtonCode(dateFieldName, thisDay, -1, "<") + xTD;
html += TD_title + DIV_title + monthArrayLong[ thisDay.getMonth()] + " " + thisDay.getFullYear() + xDIV + xTD;
html += TD_buttons + getButtonCode(dateFieldName, thisDay, 1, ">") + xTD;
html += xTR;
// this is the row that indicates which day of the week we're on
html += TR_days;
for(i = 0; i < dayArrayShort.length; i++)
html += TD_days + dayArrayShort[i] + xTD;
html += xTR;
// now we'll start populating the table with days of the month
html += TR;
// first, the leading blanks
for (i = 0; i < thisDay.getDay(); i++)
html += TD + " " + xTD;
// now, the days of the month
do {
dayNum = thisDay.getDate();
TD_onclick = " onclick=\"updateDateField('" + dateFieldName + "', '" + getDateString(thisDay) + "');\">";
if (dayNum == day)
html += TD_selected + TD_onclick + DIV_selected + dayNum + xDIV + xTD;
else
html += TD + TD_onclick + dayNum + xTD;
// if this is a Saturday, start a new row
if (thisDay.getDay() == 6)
html += xTR + TR;
// increment the day
thisDay.setDate(thisDay.getDate() + 1);
} while (thisDay.getDate() > 1)
// fill in any trailing blanks
if (thisDay.getDay() > 0) {
for (i = 6; i > thisDay.getDay(); i--)
html += TD + " " + xTD;
}
html += xTR;
// add a button to allow the user to easily return to today, or close the calendar
var today = new Date();
var todayString = "Today is " + dayArrayMed[today.getDay()] + ", " + monthArrayMed[ today.getMonth()] + " " + today.getDate();
html += TR_todaybutton + TD_todaybutton;
html += "<button class='dpTodayButton' onClick='updateDateField(\"" + dateFieldName + "\");'>Cerrar</button>";
html += xTD + xTR;
// and finally, close the table
html += xTABLE;
document.getElementById(datePickerDivID).innerHTML = html;
// add an "iFrame shim" to allow the datepicker to display above selection lists
adjustiFrame();
}
y sigue en el siguiente...
Modificar Datepicker para q empiece en lunes 
