function wszystkie() {
  frm = document.forms[0];
  cnt = frm.elements.length;
  for (i = 0; i < cnt; i++) {
    if (document.forms[0].elements[i].name.indexOf('id_page_') > -1)
      if (document.forms[0].elements[i].checked)
        document.forms[0].elements[i].checked = false;
      else
        document.forms[0].elements[i].checked = true;
  }
}

<a href="JavaScript://;" onClick="JavaScript:pform('');>

function off(id) {
  var id_p = 'id_page_' + id;

  if (document.forms[0].elements[id_p].checked)
    document.forms[0].elements[id_p].checked = false;
  else
    document.forms[0].elements[id_p].checked = true;
}

mies = document.forms['fdata'].elements['month'].value;
rok = document.forms['fdata'].elements['year'].value;

data = new Date(rok, mies, 1);
mies = data.getMonth();
rok = data.getFullYear();
dzien = data.getDate();
dzientyg = data.getDay();

function off(id) {
  frm = document.forms[0];
  cnt = frm.elements.length;
  for (i = 0; i < cnt; i++) {
    if (document.forms[0].elements[i].name.indexOf('id_page_' + id) > -1)
      if (document.forms[0].elements[i].checked)
        document.forms[0].elements[i].checked = false;
      else
        document.forms[0].elements[i].checked = true;
  }
}