Map array into multiple arrays

I've got an array with some objects in it that has a key called "day" and my problem is that, if I got more then one object that has the same value in the key "day" I need to map these objects to a new array within the orginal array.

Array

$arr = [
    {
        title: 'Hello',
        day: 1
    },
    {
        title: 'Hello2',
        day: 1
    },
    {
        title: 'Hello3',
        day: 3
    },
]

If many objects has same value in key "day", expected result:

$arr = [
    [
        {
            title: 'Hello',
            day: 1
        },
        {
            title: 'Hello2',
            day: 1
        },
    ],        
    {
        title: 'Hello3',
        day: 3
    },
]

via Chebli Mohamed