[{"month":"03","days":[{"day":"17","id":"xxx","name":"myname","lastSeen":"2025-03-17","someotherkey":"value"},{"day":"18","id":"xxx","name":"myname","lastSeen":"2025-03-18","someotherkey":"value"}]},{"month":"04","days":[{"day":"20","id":"xxx","name":"otherName","lastSeen":"2025-04-20","someotherkey":"value"}]}]

db.collection.find({"days.name":{$regex:"her"}})