[{"_id":"1a","value":10,"due_date":"20221001"},{"_id":"1b","value":5,"due_date":"20221102"},{"_id":"1c","value":7,"due_date":"20221102"},{"_id":"2a","value":10,"due_date":"20221012"},{"_id":"2b","value":7,"due_date":"20221113"},{"_id":"2c","value":8,"due_date":"20221113"},{"_id":"3a","value":10,"due_date":"20221030"},{"_id":"3b","value":9,"due_date":"20221131"},{"_id":"3c","value":11,"due_date":"20221131"}]

db.collection.aggregate([{$addFields:{day:{$toInt:{$substr:["$due_date",6,2]}}}},{$addFields:{bucketDate:{$switch:{branches:[{case:{$gt:["$day",20]},then:">20 days"},{case:{$gt:["$day",10]},then:"11-20 days"}],"default":"0-10 days"}}}},{$addFields:{bucketDateWithMonth:{$concat:[{$substr:["$due_date",0,6]}," ","$bucketDate"]}}},{$group:{/**"_id": "$bucketDate", //No grouped month*/_id:"$bucketDateWithMonth",/**With grouped month*/count:{$sum:1},value:{$sum:"$value"}}}])