>What is a real technical reason that calculating on the fly is not as good as having a table? (aside from the fact that you don't like it :))

Take a look at this great article and others in this series

https://www.sqlshack.com/designing-a-calendar-table/

Basically, every database needs to have a Dates table. It has so many business applications and solves so many problems, that you don't want to calculate things when you can have them pre-calculated already.

BTW, that particular article was recommended by a great SQL Server MVP in this thread

https://social.msdn.microsoft.com/Forums/sqlserver/en-US/76114693-d331-4dfc-9488-b14923683c9c/veterans-day-update-statement?forum=sqlgetstarted