calendar.txt says "weekdays from January to June". calendar_dates.txt says "except Christmas". Some feeds skip calendar.txt and list every single day of service in calendar_dates.txt instead: thousands of rows that say, at length, "weekdays from January to June".
It is valid GTFS. It also makes the feed bigger, slower to load and impossible to read, and it hides the service pattern from anyone looking at the data.
The law
No calendar.txt row runs on any day of the week, and calendar_dates.txt has more than 5,000 rows.
How to get pardoned
Write the weekly patterns to calendar.txt and keep calendar_dates.txt for the exceptions.