Database
[{todos:[{task:{type:"someType",},dueDate:"2021-02-28",},{task:{type:"someType",},dueDate:"2021-01-28",},{task:{type:"someType",},dueDate:"2021-03-28",},],},{todos:[{task:{type:"someType",},dueDate:"2021-01-28",},{task:{type:"someType",},dueDate:"2021-02-28",},{task:{type:"someType",},dueDate:"2021-03-28",},],}]
Query
db.collection.aggregate([{"$unwind":"$todos"},{$group:{_id:"$todos.dueDate",todos:{$push:"$$ROOT"}}},{$sort:{"_id":-1}},])