[{"_id":ObjectId("5f7720caf2b93af8d566bc7c"),"email":"blah@gmail.com","timestamp":ISODate("2020-09-29T17:59:00Z")},{"_id":ObjectId("5f7720dcf2b93af8d566ffb7"),"email":"blah@gmail.com","timestamp":ISODate("2020-09-30T01:01:00Z")},{"_id":ObjectId("5f7721bbf2b93af8d56aadc4"),"email":"yack@gmail.com","timestamp":ISODate("2020-10-01T09:58:00Z")},{"_id":ObjectId("5f771e9ff2b93af8d55c57a9"),"email":"yack@gmail.com","timestamp":ISODate("2020-09-26T04:12:00Z")},{"_id":ObjectId("5f771e9ff2b93af8d55c5f6b"),"email":"yack@gmail.com","timestamp":ISODate("2020-09-26T04:22:00Z")},{"_id":ObjectId("5f771eeaf2b93af8d55dc45c"),"email":"yack@gmail.com","timestamp":ISODate("2020-09-27T04:11:00Z")}]

db.collection.aggregate([{$group:{_id:{email:"$email",dow:{$dayOfWeek:"$timestamp"}}}},{$group:{_id:"$_id.email",dow:{$push:"$_id.dow"},visits:{$sum:1}}}])