The reason why restaurant sushi tastes better is because the restaurants use superior and quality ingredients, fresh sushi and they also have specialized skills in making the sushi and also use careful preparation techniques and have experienced sushi chefs.
Restaurants also use quality sushi and use real wasabi which contributes to the enhanced flavor and texture of their sushi.
Sushi restaurants have access to higher quality and fresher sushi and other fish as well as real wasabi roots, which offer more flavor than imitation versions.
The number one sushi restaurant in the US is the Sushi Nakazawa restaurant in New York City.
Other top sushi restaurants in the US are Uchi in Austin Texas and O Ya in Boston.
The reason why there is sometimes no sushi on Monday in some restaurants 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.