Gets or sets the day of the week.
This method can be used to set the day of the week, with Sunday as 0 and Saturday as 6.
If the value given is from 0 to 6, the resulting date will be within the current (Sunday-to-Saturday) week.
If the range is exceeded, it will bubble up to other weeks.
moment().day(-7); // last Sunday (0 - 7)
moment().day(0); // this Sunday (0)
moment().day(7); // next Sunday (0 + 7)
moment().day(10); // next Wednesday (3 + 7)
moment().day(24); // 3 Wednesdays from now (3 + 7 + 7 + 7)