Grew up Catholic. Translated this as "Friday" instead of "Thursday" on the first try and then I had to go back and correct myself.
As of my understanding, в + accusative shows a one-time occurrence, while по + dative implies a reoccurring event.
«По четвергам, он ест рыбу» – ‘He eats fish on Thursdays.’
«В четверг, он ел/будет есть рыбу» – ‘He ate/will eat fish this Thursday.’
Is that right?
Yes, that’s right.
(Also note that, unlike in English, you don't need the comma after adverbial expressions at the beginning of a sentence.)
Thank your for confirming! Спасибо большое за подтверждение!
Is the stress on the wrong part of Thursday here? I thought it was по четвергАм
It is четверга́м if you listen more closely
They fixed it! It totally wasn't before
I believe "On Thursday" (singular instead of plural) is valid because it is answering the question of which day of the week is it that he eats fish.
He eats fish on Thursday is correct.
Why is the instrumental case used?
It is Dative Plural. "По" takes Dative more often.
Instrumental is четвергом/четвергами