Translation:We generally eat fish on Friday.
Although I put 'Friday', would 'Fridays' be ok too? And in either case, could 'on' be omitted?
Yes, for habitual action, it is better to put 'Fridays'. In English, 'on' is usually needed before the days of the week to show when something happens. However, 'on' is optional after 'to be' - "The party is Friday / The party is on Friday."
Thanks for your help. Still, I guess there is such a thing as poetic licence, so just to cheer you up:
They call it Stormy Monday, But Tuesday's just as bad, Lord and Wednesday's worse, And Thursday's all (oh) so sad, The eagle flies on Friday, And Saturday I go out to play. ................................................... Sunday I go to church And I kneel down and pray
You do not need to use "the" with the days of the week, specially here, as you have a habitual action.
No, 'usually' means habitual action. So it is not just one Friday, but most Fridays. Also, days of the week do not have articles in English unless you are talking about a specific day, like "the Friday before Easter". The best way to say this is "We usually eat fish on Fridays."