Database
[{"id":"f3fd1b6c","originalVersion":"v2","timestamp":ISODate("2020-03-27T13:28:09.883+00:00")},{"id":"f3fd1b6d","originalVersion":"v2","timestamp":ISODate("2020-05-27T13:28:09.883+00:00")}]
Query
db.collection.aggregate([{$group:{_id:{"$month":"$timestamp"},idsToFetch:{"$push":"$id"}}}])