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Ah I did indeed make a spelling mistake, you were right iCake. 'в воскресенье' (or 'в воскресеньи...) didnt seem to make sense to me either.

Thanks both, resolved!

'Вы всегда ходите в ботанический сад по воскресеньям?'
'Она всегда ходить в клуб в воскресеньи?'
One means 'on Sundays, the other means 'On Sunday'. Why choose on for one sentence, and the other...