[{"_id":ObjectId("5fd289a93f7cf02c36837ca7"),"ClientName":"John","OrderNumber":"12345A","Price":10,"CreateDate":ISODate("2020-09-20T06:00:00.000Z")},{"_id":ObjectId("5fd289a93f7cf02c36837cc7"),"ClientName":"John","OrderNumber":"12345","Price":20,"CreateDate":ISODate("2020-09-12T06:00:00.000Z")}]

db.collection.aggregate([{$match:{CreateDate:{$lt:ISODate("2021-08-31T00:00:00.000Z"),$gte:ISODate("2020-09-01T00:00:00.000Z")}}},{$group:{_id:{ClientName:"$ClientName",month:{$month:"$CreateDate"}},AvgPrice:{$avg:"$Price"}}},{$project:{_id:0,ClientName:"$_id.ClientName",Month:{$arrayElemAt:[["","Jan","Feb","Mar","Apr","May","Jun","Jul","Aug","Sep","Oct","Nov","Dec"],"$_id.month"]},AvgPrice:1}}])