Database
[{"key":ISODate("2020-01-22")},{"key":ISODate("2020-01-22")},{"key":ISODate("2020-01-01")},{"key":ISODate("2020-02-21")}]
Query
db.collection.aggregate([{"$match":{"key":{$gte:ISODate("2020-01-01"),$lte:ISODate("2020-01-31")}}},{$group:{"_id":"$key"}},{$project:{"day":{$dayOfMonth:"$_id"},"_id":0}}])