The Lord's Day Is a Duty of Necessity or Mercy?

In our modern world, there are a lot of works of necessity, many of which even support the worship of God and those need to be done. I would say the same in our modern world in terms of safety professionals, police and fire, EMTs. In our congregation, we've had men who have been employed by the local water district and they would come to worship, but they'd be on call if something happened. That qualifies to fall under their category either of a duty of necessity or mercy.