[{date_order:ISODate("2020-08-09T22:00:00.000Z"),item:"banana",price:"2$"},{date_order:ISODate("2020-08-02T22:00:00.000Z"),item:"banana",price:"2$"},{date_order:ISODate("2020-08-03T22:00:00.000Z"),item:"banana",price:"2$"},{date_order:ISODate("2020-08-04T22:00:00.000Z"),item:"banana",price:"2$"},{date_order:ISODate("2020-08-05T22:00:00.000Z"),item:"banana",price:"2$"},{date_order:ISODate("2020-08-06T22:00:00.000Z"),item:"banana",price:"2$"},{date_order:ISODate("2020-08-06T22:00:00.000Z"),item:"banana",price:"2$"}]

db.collection.aggregate([{$match:{date_order:{$gte:ISODate("2020-07-27T22:00:00Z"),$lt:ISODate("2020-08-10T23:59:59Z")}}},{$group:{_id:{$dayOfWeek:"$date_order"},nbOfOrders:{$sum:1}}},{$project:{_id:0,dayOfWeek:{$subtract:["$_id",1]},nbOfOrders:"$nbOfOrders"}},{$group:{_id:null,items:{$push:"$$ROOT"}}},{$addFields:{items:{"$concatArrays":["$items",[{"dayOfWeek":0,"nbOfOrders":0},{"dayOfWeek":1,"nbOfOrders":0},{"dayOfWeek":2,"nbOfOrders":0},{"dayOfWeek":3,"nbOfOrders":0},{"dayOfWeek":4,"nbOfOrders":0},{"dayOfWeek":5,"nbOfOrders":0},{"dayOfWeek":6,"nbOfOrders":0}]]}}},{$unwind:"$items"},{$group:{_id:"$items.dayOfWeek",nbOfOrders:{$sum:"$items.nbOfOrders"}}},{$project:{_id:0,dayOfWeek:"$_id",nbOfOrders:"$nbOfOrders"}}])