Database
[{createdAt:"2020-12-30T11:26:19.948+00:00",clicks:["2021-01-27T15:58:37.109+00:00","2021-01-28T15:58:37.109+00:00","2021-01-28T15:58:37.109+00:00"]},{createdAt:"2020-12-31T11:26:19.948+00:00",clicks:["2021-01-24T15:58:37.109+00:00","2021-01-22T15:58:37.109+00:00","2021-01-28T15:58:37.109+00:00"]}]
Query
db.collection.aggregate([{$unwind:"$clicks"},{$group:{_id:{$dateToString:{format:"%d-%m-%Y",date:{$toDate:"$clicks"}}},totalClicks:{$sum:1}}}])