I have been under the impression that the keeper deadline is Sept 1st but then in one of my leagues the commish made a post saying that its really the first Monday in September which is Sept 5th
So it is the 1st or 5th?
It’s what it says on the site 
The deadline is September 1 this year.
Last year it was on September 5, but that was a one-off for launch year. Next year we may move it earlier or later to have less overlap with the baseball trade deadline.