It is my opinion that if they waited until Early/Mid December to put the wreaths on...they wouldn’t have them on in time.
There are a lot of stones.
Also, I would also assume they are meant to be there for the month or so leading up to Christmas for families and visitors.
The picture was taken in December 2018.
The wreaths are put there by thousands of volunteers across the country at their local national cemetery the weekend before Christmas.