Mongodb WeekofMonth?
The $week operator gives you the week of year as described in the docs.
The week of month can be calculated by getting the day of month and dividing by 7.
db.activity.aggregate([ {$project: { "year": {$year: "$createdAt"}, "month": {$month: "$createdAt"}, "weekOfMonth": {$floor: {$divide: [{$dayOfMonth: "$createdAt"}, 7]}} }}, {$group: { "_id": {"year": "$year", "month": "$month", "weekOfMonth": "$weekOfMonth"}, count: { $sum: 1 } }}, {$match : { "_id.year" : 2016, "_id.month" : 5}}])Note that the week of month here is 0 based. If you want it to start at 1 just $add 1. Also, the $floor operator is new in version 3.2.
Edit
You can simulate the floor using $mod (which exists in version 3.0)
"weekOfMonth": {$subtract: [{$divide: [{$dayOfMonth: "$createdAt"}, 7]}, {$mod: [{$divide: [{$dayOfMonth: "$createdAt"}, 7]}, 1]}]},