The reason why there is no sushi on Monday has to do with when fish markets were closed in the past on Sundays and so deliveries of sushi and other fish that were purchased on Monday may have been sitting in the kitchen since Friday, making the fish or Sushi less fresh.
However today you can still buy and order Sushi on Monday at many restaurants, although some smaller restaurants that sell and serve sushi might be closed on Monday as it's also a traditionally slower day for business and they often use Monday as a day off for cleaning and to allow the staff to rest.
It's not a universal rule that sushi cannot be served or eaten on Monday and many restaurants, and most particularly the restaurants with good sourcing and storage practices are often open and serve fresh sushi every day including Mondays.
Today, many of the restaurants, including restaurants that serve sushi, have very robust supply chains with deliveries occurring throughout the week, including on Sundays, so fish and sushi freshness is less of a concern than it was in years past.
Reputable sushi restaurants also prioritize fresh, high quality fish, regardless of the day, and also often will have strict handling and storage procedures.