sample: get the next week's Friday

HTML

<p id="cur"></p>
<p id="nex"></p>

JavaScript

var days = "日月火水木金土";

/*--------------------*/
/*--------------------*/

var now    = new Date();
var nYear  = now.getFullYear();
var nMonth = now.getMonth();
var nDate  = now.getDate();
var nDay   = now.getDay();

document.querySelector("#cur").innerHTML = "西暦" + nYear + "年" + (nMonth + 1) + "月" + nDate + "日" + days[nDay] + "曜日";

/*--------------------*/
/*--------------------*/

var nextWeekFriday = new Date(nYear, nMonth, nDate + (12 - nDay));

var fYear  = nextWeekFriday.getFullYear();
var fMonth = nextWeekFriday.getMonth();
var fDate  = nextWeekFriday.getDate();
var fDay   = nextWeekFriday.getDay();

document.querySelector("#nex").innerHTML = "西暦" + fYear + "年" + (fMonth + 1) + "月" + fDate + "日" + days[fDay] + "曜日";