| | Re: at the end of vs by the end of
When you use 'by' you are saying 'before time runs out'.
Hand in that report by the end of the day tomorrow.
(You have until that time or it's late/too late.)
Whe you use 'at' you are talking about that specific time.
Hand in that report at the end of the day tomorrow.
(hand it in tomorrow at 5:00, not now.)