[{shippingDate:"sunday"},{shippingDate:"monday"},{shippingDate:"sunday"},{shippingDate:"tuesday"},{shippingDate:"sunday"},{shippingDate:"wednesday"}]

db.collection.aggregate([{"$group":{"_id":"$shippingDate","days":{"$sum":1}}},{"$match":{"days":{"$gte":3}}},{"$project":{"days":0}}])