I wouldn't be surprised if a company that size (especially how big it was in the 80s) was open 24/7
I have done a quick survey on clickly keyboards and am yet to find a weekend born keyboard except for the M-13 which was a Saturday and assembled in Mexico. But this was only like 10 boards
I would have thought it would be a 24/7 operation also, but I don't know anything about union/trade/religious rules governing working hours in the US in 1989 and it is this that makes me think that there may have been some sort of blocker for working Sunday. But its just a conspiracy at this stage.
If demand does not demand it, it is good to close a factory one day a week for maintenance, clean-up, etc.
I have only had 2 factory jobs, briefly, both in the 1970s, but they generally stopped working from about noon on Saturday until Monday morning.