JSFiddle - React, Tailwind, and code Playground

HTML

<script src="//cdnjs.cloudflare.com/ajax/libs/moment.js/2.1.0/moment.min.js"></script>

JavaScript

var myIsoWeekDay = 2; // say our weeks start on tuesday, for monday you would type 1, etc.
    
var startOfPeriod = moment("2017-04-20T00:00:00");

// how many days do we have to substract?
var daysToSubtract = moment(startOfPeriod).isoWeekday() >= myIsoWeekDay ?
    moment(startOfPeriod).isoWeekday() - myIsoWeekDay :
    7 + moment(startOfPeriod).isoWeekday() - myIsoWeekDay;

// subtract days from start of period
var begin = moment(startOfPeriod).subtract('d', daysToSubtract);

console.log(begin.toString());