Let somebody know
Definition 1 of 1
Pro Tip 1/3Urgency Variation
The phrase can imply urgency. The tone and context determine how soon you expect a response. 
Please let me know ASAP if you can attend.
Pro Tip 2/3Expecting Action
It often suggests that the speaker expects some action or response from the listener. 
Let me know when you've finished the task.
Pro Tip 3/3Request for Information
It is used when asking for specific information or an update on a situation. 
Let us know your decision by Monday.