We know that you all are making trades for that final push to make AND win fantasy playoffs. If you're the league commissioner, when you create a league, you can set your trade deadline by tapping on "edit league". It'll look something like the image below.
The final time to accept a trade is after all games are played for the week of your trade deadline.
For example, if your trade deadline is the 10th week, then you can no longer accept trades after Monday Night Football is over in Week 10.