[{"date":ISODate("2022-10-21T00:00:00.000Z"),"task":{a:1}},{"date":ISODate("2022-10-21T00:00:00.000Z"),"task":{a:2}},{"date":ISODate("2022-10-20T00:00:00.000Z"),"task":{b:1}}]

db.collection.aggregate([{$match:{date:{$gte:new Date("2022-10-19"),$lte:new Date("2022-10-26")}}},{$facet:{data:[{$group:{_id:"$date",tasks:{$push:"$$ROOT"}}},{$project:{date:"$_id",tasks:1}}]}},{$addFields:{dates:{$map:{input:{$range:[0,8]},/** maybe more dynamic with $dateDiff -> { $dateDiff: { startDate: new Date("2022-10-19"), endDate: new Date("2022-10-26") }, unit: "day" } }*/in:{date:{$dateAdd:{startDate:ISODate("2022-10-19T00:00:00.000Z"),unit:"day",amount:"$$this"}},tasks:[]}}}}},{$project:{data:{"$concatArrays":["$data","$dates"]}}},{$unwind:"$data"},{$group:{_id:"$data.date","tasks":{$first:"$data.tasks"}}}])