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

db.collection.aggregate([{$group:{_id:null,data:{$push:"$$ROOT"}}},{$set:{dates:{$map:{input:{$range:[0,8]},in:{date:{$dateAdd:{startDate:ISODate("2022-10-19T00:00:00.000Z"),unit:"day",amount:"$$this"}}}}}}},{$set:{dates:{$map:{input:"$dates",as:"d",in:{$mergeObjects:["$$d",{tasks:{$filter:{input:"$data",cond:{$eq:["$$d.date","$$this.date"]}}}}]}}}}},{$project:{data:{$map:{input:"$dates",in:{$cond:{if:{$eq:["$$this.tasks",[]]},then:"$$this",else:{$first:"$$this.tasks"}}}}}}},{$unwind:"$data"},{$replaceWith:"$data"}])