MongoDB - group by month

You need to use the $month keyword in your group. Your new Date().getMonth() call will only happen once, and will try and create a month out of the string "$bookingdatetime".

db.booking.aggregate([
    {$group: {
        _id: {$month: "$bookingdatetime"}, 
        numberofbookings: {$sum: 1} 
    }}
]);

You can't include arbitrary JavaScript in your aggregation pipeline, so because you're storing bookingdatetime as a string instead of a Date you can't use the $month operator.

However, because your date strings follow a strict format, you can use the $substr operator to extract the month value from the string:

db.test.aggregate([
    {$group: {
        _id: {$substr: ['$bookingdatetime', 5, 2]}, 
        numberofbookings: {$sum: 1}
    }}
])

Outputs:

{
    "result" : [ 
        {
            "_id" : "03",
            "numberofbookings" : 1
        }, 
        {
            "_id" : "07",
            "numberofbookings" : 1
        }, 
        {
            "_id" : "10",
            "numberofbookings" : 1
        }
    ],
    "ok" : 1
}