[{"_id":ObjectId("5fd289a93f7cf02c36837ca7"),"ContactName":"JohnSmith","CrtDate":ISODate("2021-04-21T13:59:49.569Z"),"UpdDate":ISODate("2021-04-21T13:59:49.569Z"),"activities":[{"activityTypeDesc":"Type1","dateOfActivity":ISODate("2021-02-20T06:00:00.000Z"),"activityNote":""},{"activityTypeDesc":"Type2","dateOfActivity":ISODate("2021-03-04T06:00:00.000Z"),"activityNote":""},{"activityTypeDesc":"Type3","dateOfActivity":ISODate("2021-01-04T06:00:00.000Z"),"activityNote":""},{"activityTypeDesc":"Type4","dateOfActivity":ISODate("2021-04-15T05:00:00.000Z"),"activityNote":""},{"activityTypeDesc":"Type5","dateOfActivity":ISODate("2021-03-31T05:00:00.000Z"),"activityNote":""}]},{"_id":ObjectId("5fd2ca65d1a01d157c0179be"),"ContactName":"ScotJohns","CrtUserId":"JohnSmith","CrtDate":ISODate("2020-08-07T05:00:00.000Z"),"activities":[{"activityTypeDesc":"Type1","dateOfActivity":ISODate("2020-12-15T06:00:00.000Z"),"activityNote":"ProptoOCC"},{"activityTypeDesc":"Type2","dateOfActivity":ISODate("2021-04-17T06:00:00.000Z"),"activityNote":"txreg"},{"activityTypeDesc":"Type3","dateOfActivity":ISODate("2020-12-01T06:00:00.000Z"),"activityNote":"TestNotes"}]}]

db.collection.aggregate([{$match:{$and:[{"$expr":{$anyElementTrue:{$map:{input:"$activities.dateOfActivity",in:{$and:[{"$gte":["$$this",{"$dateFromString":{"dateString":"2021-03-01T00:00:00.0000000Z"}}]},{"$lt":["$$this",{"$dateFromString":{"dateString":"2021-04-20T00:00:00.0000000Z"}}]}]}}}}}]}}])