Below is a photo from the Atlantic Yards webcam, from 6:10 am (note time stamp upper left).
However, as noted in the most recent Construction Alert (bottom), covering this week and next and dated 5/25/15, there's no mention of weekend work on the green roof.
Note that normal construction hours are weekdays after 7 am, until 6 pm. An After Hours Variance is required if there's a desire to perform construction activity before 7 am, after 6 pm, or on the weekend, according to the Department of Buildings.
Maybe they got a variance, maybe not. But even if they did, they didn't disclose it to the public.
That reflects an ongoing frustration regarding the Atlantic Yards/Pacific Park project: lack of transparency.
The Construction Update does contain some CYA text: Please note: the scope and nature of activities are subject to change based upon field conditions.
Still, as shown with belated disclosure of other activities, that still should prompt some announcement.