The curfew has been in effect every summer that I know of since 2000. It is only during the summer months. I'm not sure what time of day it goes into effect, but I do know that the 8:45 pm DFW-LGA and 9:30 pm ORD-LGA flights do not operate over the summer because of the ban. It makes flying from western cities into LGA a problem if your looking for an afternoon departure.

Just another reason JFK is superior to LaGuardia in every way, shape and form.