[{dates:["2019-01-29","2019-01-29","2019-01-29","2019-01-29","2019-02-06","2019-02-06","2019-02-06","2019-02-08","2019-06-04","2019-06-25","2019-07-26","2019-08-15","2019-08-15",]}]

db.collection.aggregate([{$unwind:"$dates"},{$group:{_id:"$dates",count:{$sum:1}}},{$group:{_id:null,avg_day_count:{$avg:"$count"}}}])