Conventions have always been "cash cows" for the WTS. Even more so since they've utilized the Assm Halls for many of them. (NO rental fees!) But even at rented facilities, they planned conventions when the facilities were in low demand. Here in the US, there were many convention over the week that had 4th of July, as all non-JW's are involved in holiday activities and convention centers sit idle -- that translates as CHEAP! At some venues, they even bartered for lower (or free) rental rates by having their free labor force to an annual "deep cleaning" or by providing set up and tear down for other events.
