It may well be an oversight, but not for the reason you might imagine. Friday with no preposition is an American usage that the course writers may have overlooked. on Friday is correct British English. Clearly the American version needs allowed (as we say in Scotland).
I don't know what is correct across the pond but in the UK I would say that on Friday is OK in the future but incorrect in the habitual. I would say
I do not eat meat on Fridays.
I will not eat meat on Friday.
I was offered two alternative options: feòir or feòil.
(1) Both were grammatically correct (although the use of the genitive in feòir is a bit old-fashioned); (2) both made sense; and (3) both were logically possible - so it seemed an odd question. However, only one was true (as I am neither a vegetarian nor a Christian) so I went with feòir but was marked wrong. Surely wrong answers should fail one of the three criteria above?
That made me think. We know Gaels used to fast on Friday - and on Wednesday, but not on Thursday:
- Dihaoine - Di [na] h-aoine - 'day [of the] fast' (Friday)
- Diciadain - Di [na] Ciad Aoin(e) - 'day [of the] first fast' (Wednesday)
- Diardaoin - Di [ead]ar [an] d[à] Aoin - 'day between [the] two fasts (Thursday)
So why did Presbyterians cease to fast - if indeed they did? This article provides a comprehensive analysis.
See also Fasting and Abstinence in the Catholic Church. D
The difference you don't get is to do with complexities of English and transatlantic confusion so should not be distracting us from learning Gaelic. As far as I am concerned they should accept all the main option in English
I do not eat/will not be eating meat (on) Friday(s).
This gives us 8 permutations altogether and there is nothing to be learned about Gaelic from saying that some of these are more logical than others in English.