How can I repeat a command once for each file?

You can always type in the names of the files you want to process when writing the loop, but it's usually better to use wildcards. Try running this loop in the console:

for filename in seasonal/*.csv; do echo $filename; done

It prints:

seasonal/autumn.csv
seasonal/spring.csv
seasonal/summer.csv
seasonal/winter.csv

because the shell expands seasonal/*.csv to be a list of four filenames before it runs the loop.