A lot of these days are paid public holidays that JWs feel no personal obligation towards. Perfect conditions for getting car groups together and finding people at home. A dick move, for sure, but perfectly logical for a contrarian group who have issues with mainstream churches and government.
Historically, it wasn't just about public holidays. In 30 Years A Watchtower Slave, Schnell says that they would deliberately have field service on a Sunday at times when the mainstream churches would be holding their services. That way they could find the disillusioned churchgoers (American society was much more church orientated in his time) and persuade them to give the JWs a chance. I think they held their Kingdom Hall meetings at non traditional times on the Sunday, such as during the afternoon.