it is called setDate , i know, a bit confusing haha
var today = new Date() // today is 19/09/2016
console.log(today.getDate()) // will print 19
today.setDate(20)
console.log(today.getDate()) // will print 20
var today = new Date() // today is 19/09/2016
console.log(today.getDate()) // will print 19
today.setDate(20)
console.log(today.getDate()) // will print 20
assuming there was a setDay, and you invoke:
today.setDay(3)
should today now be: 21/9/2016 or 14/9/2016 or 19/10/2016 or 19/9/2040 or ... ?
I think you can see that unlike the other setters, setDay would not have an "obvious" implementation.
getDay returns 1 (Sunday=0, Monday=1, ....)
getDate returns 19