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Convert field from string to date -

db.post.find().forEach(function(p){p.postdate = new Date(p.postdate);db.post.save(p);});

Aggregate by that field -

db.post.aggregate([{$project:{day:{$dayOfMonth:'$postdate'},month:{$month:'$postdate'},year:{$year:'$postdate'}}}, {$group:{_id:{day:'$day',month:'$month',year:'$year'}, count: {$sum:1}}}])