Update 2020
There are now multiple ways to achieve this.
1) Using the day
method to get the days from 0-6:
const day = yourDateObject.day();
// or const day = yourDateObject.get('day');
const isWeekend = (day === 6 || day === 0); // 6 = Saturday, 0 = Sunday
2) Using the isoWeekday
method to get the days from 1-7:
const day = yourDateObject.isoWeekday();
// or const day = yourDateObject.get('isoWeekday');
const isWeekend = (day === 6 || day === 7); // 6 = Saturday, 7 = Sunday